A vampire is a being that sustains itself by feeding on the life essence of other creatures. It is most commonly blood that they feed upon, and it is usually sucked out using the vampire's fangs to bite a victim's neck. They are often portrayed as having a rivalry or war going on with werewolves.
Due to the large increase in their popularity during the last couple of decades, there is now a massive number of novels, games, TV shows and movies that feature vampires, leading to many different species of vampires in existence. Thus, there are many varieties, differing in appearance, abilities, and traits.
Vampires in Fanfic
Vampires can be found inserted into most fandoms (often for little reason other than the fangirl's will), sometimes even being blatantly ripped out of another fandom. When vampires are introduced into a story, it will usually lead to angst. Since the release of Twilight, the chance of vampires (especially angsty ones) appearing in stories has increased dramatically, as well as the potency of their angst, leading to wangst. They are usually depicted as vicious antagonists or angsty protagonists.
There is also often an element of romance found in 'vampire-AU' fanfics. Holding their victims' lives entirely in their hands, vampires can be seen as dominators (or semes, in yaoi) in a fetish-like manner.
Vampires in Badfic
Although they are very cool and usually have more enhanced physical capability than the average person (possessing supernatural abilities, super powers, enhanced senses, and immortality), a great deal of protagonist vampires are angsty or broody, even when there is no valid reason to be this way. Many follow the example of Twilight and assume that most vampires who don't (or won't) prey upon people are inherently unhappy with their existence.
Making a vampire immune to common ways in which they are killed without an extremely plausible explanation is a charge. Other common charges include changing their appearance and giving them powers/abilities not found in their continuum. Taking vampires from one continuum and putting them in another is also a charge, and it is a very bad idea. When going into a fic involving vampires, even basic knowledge of the canon's vampires is highly recommended, as proper preparation is never a bad thing.
Vampires in the PPC
The following are vampire agents: