Vampires in D&D are one of the most insidious types of undead, brilliant masterminds who dominate entire towns and nations, bringing them under their control and using them as a personal feed bank. They can transform into bats, or call bats, rats, or wolves to aid them, and can also evaporate into a cloud of gas in order to escape. They can charm people simply by meeting their gaze, and they create vampires by killing others. Their attacks drain levels, as well.
Of course, vampires have several specific weaknesses, too. Holy symbols, mirrors, and garlic are things they have a severe aversion to, and they must sleep in a coffin during the daylight whose bottom is covered in soil from their grave.
Vampires don't look very dead. Though pale and drawn, and often with a fierce, predatory look in their eyes, and though they do possess fangs, they can largely blend into a human populace, if they so choose. They look very similar to how they appeared in life.
Vampires first appeared in OD&D's White Box, where they had a potent weapon drain, allies in rats, bats, and wolves, transformation powers, and a host of other abilities and weaknesses (including the level-draining attack).