Hellblazer features a number of different supernatural and otherworldly creatures. The following are the most prominent forms, though others exist.
Angels
Angels are the messengers of the Judeo-Christian God, but that doesn't necessarily make them 'good guys'. In fact, angels are usually depicted as lacking any empathy with human beings and being generally indifferent to individuals. If they care for man at all, it is as a species alone.
Because their god is worshipped by various different faiths, a given angel will operate under many names. In Hellblazer issue 64, it is revealed that Gabriel performed all of the duties attributed to him by the Christian, Jewish and Islamic faiths.
Angels have a 'true' form that is only rarely glimpsed - one of fiery light. However, they are most usually depicted in one of two forms: the traditional angel figure with flowing, feathered wings, or a human form with which they can interact with the human world. They are almost uniformly masculine in appearance, though they may lack genitalia. Some of them are still able to impregnate women, however - in Hellblazer issue 64 it is implied that Gabriel was the one who impregnated the Virgin Mary, and in Hellblazer issue 61 an angel successfully impregnates a succubus.
Severing an angel's wings will render it mortal, if still awfully hard to kill. One sure-fire way of killing an angel is to crush its heart (Hellblazer issue 80. To this end, removing an angel's heart and keeping it in a secure place will compel it to do your bidding.
Notable angels: Gabriel, Tali. See also: List of minor angels.
Fallen angels
After God created man and demanded that the angels bow down to him, the angel Lucifer rebelled and one third of the angels followed with him. A war in Heaven ensued, which was won by God's angels. As punishment, Lucifer was cast down into Hell, as - presumably - were the other surviving angels, though their whereabouts remain unknown. Fallen angels remain angels and should not be mistaken for demons.
Notable fallen angel: Lucifer.
Demons
Demons come in all forms, from the almost-human to the monstrously grotesque. They are perfectly capable of changing their shape to suit their needs but generally settle on one consistent look. They can have varying powers, but are generally shown to be fast, strong and deadly.
They cannot be killed outright unless one is wielding The Ace of Winchesters, but they can be damaged sufficiently that they are forced to spend years rebuilding themselves. They can also be bound if properly summoned and trapped by the use of their true name. John Constantine's first major failure was when he summoned the demon Nergal but used the wrong name, resulting in him taking the life of a child (Hellblazer issue 11).
Demons can be born through the normal route of procreation, whether with another demon or with a human (Hellblazer issue 200), but many demons are Hellbound souls that have shown sufficient 'promise' (ie. a particularly nasty streak) and have been subsequently transformed into true demons.
Most demons in the Hellblazer series are shown to be based out of Hell, where they lurk and plot their schemes. In fact, demons have a fully-functioning society in the inferno, complete with stock market (Hellblazer issue 3 and 250), entertainment (Dark Entries) and a hierarchy largely based on 'might is right' (Hellblazer 209).
Not all demons are based in Hell, however. The concept of 'demons' features in many religions, which means - thanks to the polytheistic world of Hellblazer - that they also exist in many non-Juedo-Christian forms. For example, Papa Midnite was surprised to discover that the hunger demon, Mnemoth, was of African origin, not Hell. The First of the Fallen is also counted as a demon, although he was never a human and is not of the actual demon lineage. Demons are distinct from fallen angels.
Notable demons: Nergal, The First of the Fallen. See also: List of minor demons.
Succubi
Succubi are a specific sub-set of demon and, as you might expect, reside in Hell. They are children of Triskele the Wyrm Queen, although given that she is a human head on a long, snake-like skeletal body that may be a figure of speech. She is certainly their leader and a figure of fear for them.
Although their true form is that of a hulking, winged, leather-skinned demon, succubi usually take the form of beautiful young women so that they can seduce humans. Succubi can take on any form they like - the better to tempt their prey - but usually settle on a single consistent appearance that becomes their default form.
Succubi subsist primarily (if not exclusively) through sex, absorbing some of a human's life-force. It is possible for a succubus to kill a man completely during the act, reducing him to a burned-out skeleton (as seen in Hellblazer issue 187), but it's also within their capabilities to let the man go with no ill-effects. Indeed, in that issue John says that succubi don't usually feed like that. John himself slept with the succubus Ellie in Hellblazer issue 104 in order to restore his demonic taint; it is unknown whether this would happen to any man that slept with a succubus.
Succubi can become pregnant, though this may happen only with other supernatural species; the only two examples we have seen have involved succubi being impregnated by an angel (Hellblazer issue 60) and by a vampire (Hellblazer issue 187).
Perhaps because they spend so much time in the mortal world, and because their pleasure is derived from sex rather than murder, succubi are usually portrayed as being more 'human' and compassionate than other demons.
Notable succubi: Ellie, Gloria, The Wyrm Queen.
Elementals
When the Swamp Thing rose from the Louisiana bayous, it believed that it was Alec Holland, a brilliant scientist transformed into a plantlike monster. What it discovered, thanks to the intervention of John Constantine, was that it was actually an elemental with the memories of Alec Holland.
Elementals are spirits that represent certain facets of existence: fire, earth, wind and water being the obvious four, but there are also plant elementals - like Swamp Thing - and flesh elementals like his daughter, Tefé Holland. They are, apparently, a naturally occurring force of nature; in the comic book Lucifer, the titular character created his own universe and his own Earth, which spontaneously generated its own plant elemental.
Although elementals are generated by the forces of nature as and when they are required, they need a consciousness to guide them properly. For this reason, whenever a plant elemental needs to be created, a human being (or whatever the dominant life-forms are; dinosaur plant elementals once existed) must die. The human's soul passes on to the afterlife, but its conscious mind is copied and adopted by the elemental and used as the basis of its being.
Elementals are connected to every instance of their element in the world on a quantum level (for plant elementals this is called The Green; for flesh elementals it is The Red, etc). They can use this to control their element (eg. controlling a creeper's vines to ensnare an enemy) or even to instantaneously transfer their consciousness to an element on the other side of the planet and use it to grow a new body.
Notable elementals: Jack of the Green, Swamp Thing, Tefe Holland.
Ghosts
Ghosts are the souls of the dead that have, for whatever reason, not moved on permanently to the afterlife. They are able to take any form they please, although the remains of their human ego means that they're most likely keep the appearance that they had in life - sometimes including the wounds that killed them.
There is no one standard form that a ghost may take. Some come back as invisible poltergeists. Others are visible but intangible, or only visible to those they wish to haunt. Still others can take on a solid form and interact with the real world, possibly giving the impression that they are still alive.
Ghosts also have no consistent range of abilities; some can possess living humans (Hellblazer 182) while others can force them to relive the memories of the lives they once lived (Hellblazer issue 71 and Hellblazer issue 106).
Not all ghosts are human - John Constantine has encountered the ghost of an animal on at least one occasion (Hellblazer issue 98).
While most ghosts are on Earth because they cannot or will not move on, it is possible for a spirit to move on to the afterlife but return to Earth to give advice or just to experience the world once more. Brendan Finn came down from Heaven briefly to visit John when he was in Dublin (Hellblazer issue 76). Meanwhile, a group of pagan ghosts broke through into the world in Hellblazer issue 182.
It is possible to help a ghost move on in several ways: by performing a ritual (Hellblazer issue 77), by confronting their killer (Hellblazer issue 250), by giving them a proper burial (Hellblazer issue 71) or through many other means besides.
Gods
Hellblazer and its related comics exist in a pantheistic universe that includes gods from all religions. Gods are created when human belief coalesces into physical form, and they continue to thrive as long as someone believes in and worships them. If they no longer have worshippers they may continue to siphon belief from icons of beings that are worshipped (Lucifer issue five), prayers that are sent out without a specific target and fall into their area of worship (The Sandman issue 23), or celebrations of what they represent (Hellblazer issue 49).
Belief can cause gods to manifest retroactively, changing the past. Where beliefs contradict each other, all are true (eg. the sun is a ball of fire being pulled by a chariot and being rolled across the sky by a beetle and a star). However, it seems that the belief which has the most adherents becomes the 'true' religion for that world, thus at present the Judeo-Christian-Islamic god has precedence over reality. However, when on another god's home ground (ie. their afterlife or kingdom) even a powerful god can be rendered impotent.
Notable gods: The Lord of the Dance. See also: List of minor gods.
Vampires
The vampires seen in Hellblazer conform to most modern interpretations: they are beautiful, predatory, immortal and subsist on human blood, which they drink from victims using their retractable fangs. They also have the ability They seem to live and hunt in packs, but have developed a society and hierarchy, with their leader being deemed The King of the Vampires. While their attitude to humans is usually shown to be sociopathic, they are capable of building close emotional bonds with other vampires.
They are also presumably capable of falling in love with humans - in Hellblazer issue 142, Constantine recalls a former girlfriend who contracted HIV but married a mysterious aristocrat and now doesn't age any more. Assuming she did indeed get turned into a vampire, it's possible that vampires are not inherently murderous and that Constantine only usually encounters the worst of them
It is almost impossible to kill a vampire - even decapitation doesn't work - unless you expose them to direct sunlight. An alternative method of incapacitation is to bring them into contact with demon blood, which has a corrosive effect on their tissue.
Although they are dead, it seems that vampires are capable of having children: some time prior to Hellblazer issue 187, a vampire successfully impregnated a succubus, who gave birth to children that could change shape between human and bat-like form.
Notable vampires: The King of the Vampires. See also: List of minor non-humans.
Were-animals
Were-animals are, like the werewolves of legend, human beings who take on the form of animals, either intentionally or due to outside factors - like a full moon. Weres don't necessarily transform fully into their given animals - some become wolfmen-like anthropomorphic creatures that are even capable of riding horses and using tools.
They also have varying levels of control once they transform - some become uncontrollably ravenous, while others maintain all their human faculties. Animals that have been mauled by werewolves have a distinctive smell about them - a metallic smell that's not unlike chewing tin foil (Hellblazer issue 109).
The origins of lycanthropy are unknown, but the explanation of one strain can be found in Hellblazer issue 109, which explains that a virus that emerged from nowhere 3,000 years ago caused these strange mutations and also granted the infected immortality. The infection is transmittable through saliva - even if it's been left on a half-eaten animal.
Not all were-animals are wolves; Constantine worked alongside a werecat in a storyline that began in Hellblazer issue 85, though whether the cat was real or the result of a reality-warping spell is unknown.
Notable were-animals: Brother Chalice, Murnarr See also: List of minor non-humans.
Zombies
There are two kinds of zombie in Hellblazer. The first is the traditional George Romero-style 'living dead' type: a corpse that is reanimated no matter how severe its level of putrefaction. These can be mindless walking corpses or - more often - decaying bodies into which the original soul has been returned (Hellblazer issue 258). Generally the term 'zombie' is reserved for people who have been brought back from the dead after the body has started to decay - if only for politeness' sake.
The second type is the voodoo 'zombie', as seen in Hellblazer issue 1. The creation process is unknown, but if it's anything like the real world then it involves dosing the victim with a neurotoxin that causes brain damage and makes them pliable and easy to control. Papa Midnite creates this kind of zombie to act as a servant, and also as the subjects of the blood sports he hosts for paying crowds.