A riff is a repeating phrase which forms a 'hook' for the listener. It can appear in intros and outros, though sometimes riffs are played between parts of a song or even throughout verses and choruses.

A lick is a learned melodic line which is generally used in a solo or lead break. It generally only appears once or twice in a piece of music, and doesn't form the central 'hook'

Really the only difference is that riffs are played over and over, and licks aren't, so it's fair to say that all riffs are licks, but not all licks are riffs.