Quote Originally Posted by Wikipedia, condensed by Reli
In music, syncopation includes a variety of rhythms that deviate from regularly spaced strong and weak beats in a meter (pulse). These include a stress on a normally unstressed beat or a rest where one would normally be stressed.

If you want a visual, the trebel is syncopated, bass is not. The quarter note comes on the second half of the beat, not on the beat like it does below.