With every type of beast, there is someone out there who is willing to chase after it and kill it, perhaps even for a price. Some hunters may be actively involved, or simply do research and weapons supply.
With Vampires, the terms is generally Slayers. They hunt with vast amounts of silver and garlic, with wooden stakes and knives part of their arsenal. There can be either singular or paired hunters, usually close family/friends, or there can be a large company or group of them, usually within some government sect or building under some strange psuedonym. But the ultimate way to kill a vampire is to cut off the head.
Werewolf hunters are a little different. The have more guns in their arsenals, with the common silver bullets or stakes which can kill them. Seeing as how werewolves nowadays have become more radically cool and less or more easy to kill. But to say the least, werewolves may be more aggressive, but they can die in more human ways than vamperic ways. Get hit by a car, the former is more likely to splatter and die. Though the ultimate, yet disputed way, is to kill them with a silver bullet through the heart.
Demon hunters are more radical than most in their techniques. Demons in general stick more to the shadows than other monsters. The darkness hides them well, so many a hunter goes to people who are possessed or haunted by demons and helps to 'Clense' people and their homes, as the term goes. Some ways to kill demons which have become physical, emulating an anthropomorphic form are to use a demon bomb, an Angel's blade/sword, or the touch of an angel.
For hunter references, try: Blade, Buffy the Vampire Slayer, Soloman Kane, the Grimm Brothers, Vampire hunter D, the Lang family, Constantine, Devil May Cry, Poltergeist, Dogma, various manga/anime sources, The Men of Letters
