I think of a "lick" as a piece, a section of a song or solo... it may only occur one time, though often it occurs more than once in a song or solo, not necessarily in constant repetition.

I think of a "riff" as a repeating piece, like, say, a repeating chord progression or series of notes, sort of like a repeating loop.

By that definition, I guess a "riff" could be a "lick" that repeats... but not all licks are actually riffs...

That's how I understand the two.