Guitar Riff, Guitar Lick, the two terms are used  interchangeably. But, what exactly is a Guitar Riff? And, what is it that makes a, "Guitar Lick" different from a "Riff?"

GUITAR RIFFS:
A riff is thematic. It generally serves as the main musical idea for a (section of a) song. Riffs are often repeated and are also developed further in a piece, sometimes with variations, sometimes in different keys, but always recognizable as the same main musical idea.

Because a guitar riff is a main theme for a song, it often becomes inextricably associated with that song --- if you heard the riff out of context (say, someone trying out guitars in Guitar Center), you'd associate it with the song.



Think of songs like; "Kashmir", "Smoke On The Water", "Crazy Train," or "Smells Like Teen Spirit". If the song is a hit, the riff becomes quotable, and anyone else who plays the riff is making an allusion to the original song.

Kashmir:



 Crazy Train:



GUITAR LICKS:
A guitar lick is musical idea, just like a riff in many ways, but often guitar licks are incomplete. A lick might be just a fragment of a guitar solo, or only a small portion of a riff. By itself, a guitar lick doesn't usually become thematic.

A guitar lick that forms a theme within a piece of music essentially becomes a riff within that piece. A guitar lick will also tend to combine itself with other licks to form more complete musical ideas and thus giving us a "Guitar Solo."




Because a guitar lick isn't the main theme of a piece, it doesn't have that same association with the song, and so it's very often transferable: it can be used in other songs without necessarily having to allude to the original song's idea of usage.

For example, the blues is full of standard licks. Take, for example, the first lick played by Stevie Ray Vaughan in "The Sky Is Crying" and compare it to the lick Hendrix plays in "Red House" (@ 2:14). It's practically the same, but because it's not thematic, it's more an indicator of style than a direct musical quote.

The Sky is Crying (opening lick):



You can play this lick whenever you want and no one will think you're quoting Hendrix. At most, they'll think you've listened to a lot of Albert King.

"This Charming Man" by The Smiths opens with a lick that, because it forms the main musical idea of the song, is also a riff. If you play this riff, people will assume you're quoting Johnny Marr.

This Charming Man (opening lick)



The beginning of Clapton's solo in "Sunshine of Your Love" (@ 2:02) is a lick, but because it's also recognizably the riff from "Blue Moon", it becomes a quote.

Overall, the point is that the difference between a riff and a lick has more to do with the roles they each play in the song than whether or not they involve chords - scales - or both.