The past thirty years have seen musical genres splinter, creating a world of new sounds and hybrids. During the past five years alone, the advent of mp3 players, Napster, and digital downloads has subdivided even albums themselves. We've entered a period in which individual songs are more popular than full albums, where a single mp3 can turn a band into tomorrow's next big thing. The Pitchfork 500 will include lively, insightful reviews of the 500 most important songs released between January 1977 and December 2006.