| A riff is a piece of music that repeats.
A song generally is played verse then chorus then verse and it repeats. In some songs, in between the verses and the chorus's a peice of music is played to make the change from the verses to the chorus's, and visa versa, smoother, and increase the time between the verse and chorus. Thats called the 'Bridge' ie, it links (or 'bridges') two peices of music.
ie, instead of playing the chorus music and then instantly changing back to the verse music, a peice of music might be played in between to make the change sound smoother, and also to space out the length of time between the chorus and the verse. | 
