A vampire hunter or vampire slayer is someone who specializes in finding and destroying vampires. In some incarnations they may deal with other dangerous supernatural creatures as well.
Vampire hunters are humans who kill vampires for a living - often because of revenge or extreme religious beliefs. There have been examples of mages and vampires as hunting fellow supernatural creatures as well as examples of bounty hunter characters who hunt vampires for profit.
These hunters are often imagined as vigilantes and avengers - compelled to kill vampires because of personal loss or divine inspiration. There is a wide range of origins -- from the dark stranger sweeping in to save the townspeople to the untried locals rising up to defend themselves and a whole lot more.
The organizational strength of vampire hunters varies. Having vampire hunting as a familial tradition handed down through generations of a bloodline is popular in vampire fiction. Most hunters seem to work alone or within a network of other hunters but there are vast organizations with access to various troops and weapons.
Hunters must be well versed in vampire lore and effective weapons to take advantage of the vampires' traditional weaknesses. These have included a range of things from firearms with specialized ammunition, appropriate religious symbols, and modified crossbows with wooden stakes to water guns filled with holy water and store-bought garlic powder. Physical trainings is important as well as vampire hunters would have to be almost preternaturally fit to defeat a creature of the night. Varied methods such as martial arts and stealth training can be seen in many incarnations of vampire hunters.