Syncopation is a rhythmic technique achieved by placing the stress, or accent, on a normally unstressed beat. Rhythm functions on a series of stressed and unstressed beats that drive the underlying meter. The time signature dictates which beats are to be stressed; for example, typical 4/4 time stresses the first and third beats, while 3/4 stresses only the first. Syncopation would place the stress on a beat other than those mentioned; in 4/4, syncopation might put the stress on the second and fourth beat, for example. Anytime an accent is played on a beat other than that traditionally stressed (regardless of instrument, though drums in pop music usually dictate this), it's syncopation.
Syncopation isn't only found on the straight unstressed beats of a song, however. Often, syncopation will be achieved by placing the stress on the upbeat. Tap your foot in 4/4 time; the point at which your foot is in between taps is the upbeat. This type of syncopation is a half-step off from any defined straight beat and can be used to create energy within a song. Traditional polka and modern ska (a reggae-infused style of rock music) often employ upbeat syncopation within songs; in fact, upbeat syncopation has become the main defining characteristic of modern ska music.
Playing or singing a note just before or after the beat also constitutes syncopation, though this is a trickier sort. Because there's no defined beat on which the syncopation can fall, it takes great care on the musician's part to avoid making this sound like an accident. This type of syncopation requires a natural sense of rubato for the piece in question and can add miles of space to the song's character if used correctly. Rhythm & blues and soul music often use this sort of syncopation, and it has spread to be included in the rhythmic lyrics of many hip-hop songs.
