Snakes in Western culture generally represent some kind of insidious threat—think "snake in the grass." This idiom is an example of how snakes are perceived culturally, and there are other examples in language, too, such as the connotations of describing someone as "a snake." This can be traced to the Biblical story of Satan as the snake, or serpent, in the Garden of Eden who tempted Eve to sin.
This association between snakes and evil can be found in popular culture in lots of places. In the Harry Potter series by J.K. Rowling, for example, those who can speak to snakes—"Parseltongue"—are viewed with suspicion, and the symbol of Slytherin House is a snake. Even the name "Slytherin" is an allusion to snakes. While Slytherins are not described as being outright evil, they are self-interested, devious, and certainly have the potential for evil. Lord Voldemort, the villain of the series, has a forked tongue like a snake.
Snakes have symbolized evil in the West since before Christianity came to Europe, too. In Norse mythology, Jörmungandr is the serpent who will kill the god Thor at Ragnarok, the end of the world. European mythology is also rife with serpents, sometimes called "wyrms" or "dragons," who capture princesses and must be killed by a knight (think George and the dragon).
However, there are also cultures in which snakes have a completely different connotation. Some Native American cultures, for example, such as the Hopi people, see snakes as representative of fertility and renewal.