That's it. I was wondering what is the difference between a riff and a lick.
The Wikipedia page for lick is somewhat confusing-- what do you think?
A riff is thematic. It serves as the main musical idea for a (section of a) song. Often it's repeated and developed, sometimes with variations, sometimes in different keys, but always recognizable as the same main musical idea. Because a 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 "Kashmir", "Smoke On The Water", 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.
A lick is musical idea, too, but often it's incomplete. It might be a fragment of a solo or a portion of a riff. By itself, it doesn't usually become thematic---in fact, a lick that forms a theme essentially becomes a riff. A lick combines with other licks to form a complete musical idea. Because a lick isn't the main theme, it doesn't have that same association with the song, and so it's transferable: it can be used in other songs without necessarily having to allude to the original.
Here's are a few examples of what I mean:
FWIW, I disagree with the idea that licks are necessarily single-note phrases. For example, check out Joe Pass's solo in Sarah Vaughan's "I've Got The World On A String"--- the lick I'm referring to comes at 3:01. It's chordal, but still very much a (delicious) lick.
My 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.
I doubt that this is an accepted difference, but I usually envision a riff as an integral part of a song, often repeated multiple times throughout the song. Could be individual notes mixed in with chords, or just individual notes, but it's an important part of the song, and you'd have to learn it to make the song sound correct.
A lick (again, in my definition) is, as Dr Mayhem said, something used in solos, used by experienced improvisers to give their solos structure, and by inexperienced improvisers to avoid having to make TOO many on-the-fly decisions. The bit differentiation in my mind is that a lick can be changed or removed from the overall work, and the work is still fundamentally the same. If you remove a riff from a song, it doesn't sound the same, and even a layperson may notice.
I think that the Wikipedia page you linked to has a nice simple summary:
"A lick is "a stock pattern or phrase"  consisting of a short series of notes that is used in solos and melodic lines and accompaniment."
"A lick is different from the related concept of a riff in that riffs can also include repeated chord progressions. Licks are usually associated with single-note melodic lines rather than chord progressions."
I think that 'It smells like teen spirit' has caused a lot of confusion here. Leaving aside 'lick,' the opening chords of the Nirvana song are not a riff. It is simply the chordal base to the song even if it is instantly recognisable. I saw a UK BBC TV programme last week on riffs which included very few indeed. If you want riffs try Wishbone Ash. The idea that anything of the noodling Johnny Marr does is a riff contradicts the Smiths' whole aesthetic. Most classic riffs are very simple short (Dead Meadow for longer ones) pentatonic minor memorable phrases over one chord (like Voodoo Chile) usually doubled by the bass. Everything else is just guitar playing.
I would diffrentiate by calling a riff a musical idea usually in the lower register serving as a main part of the tune, usually single notes but not exclusively think about Mr. Brownstone. But definitely not all chords. For instance I would never call Smells Like Teen Spirit a riff. A lick is something more likely to be in the upper register as an augmentation to the main tune. Think of those high notes in the intro and verse part of back in black. The main diffrentiation though is whether its the main tune or not. The high vs low is just an accidental. For instance the low notes played in Back In Black are still licks and not riffs.