"A riff is a short, repeated, memorable musical phrase, often pitched low in the guitar, which focuses much of the energy and excitement of a rock song."

A "lick" is a short musical phrase in between the rhythm parts, sometimes repeated but not as a rule.

The classic example of a riff would be the part you think of when you hear Smoke On The Water by Deep Purple. Another more modern example would be the part you think of when you hear Enter Sandman by Metallica. The short solo-like phrases interspersed between the riff is usually what is considered a "lick".

In short, a riff is a rhythm line and a lick is a lead line.