Good description @Socio. To me a riff is a rhythmic part and a lick is a phrase in a lead part.
@Richard_close2u Justin does describe riffs and licks over a few different lessons although itās not a clear defining point. But the beginner grades are definitely focused on chords and strumming/fingerpicking, not riffs or licks.
All I might see slightly differently is āback up partā. If I think about riffs, those classic most recognisable riffs, then āback upā doesnāt really do it for.
Iām thinking contenders for the most recognisable of all time, dare I say GOAT riffs (have a natural aversion to all this GOAT talk). For example Smoke on the Water, Satisfaction, and (to show Iām not totally into boomer grandad rock) Seven Nation Army.
ā¦ still over 20 years old 
You got me, still dad rock, not getting into kiddie rock (is that an oxymoron?).
Absolutely, I presume it all depends on the context in the song.
Interestingly, I watched this youtube video this morning and f I recall correctly it covers smoke on the water with regards to licks and riffs.
Riff ā¦
a repeating and recurring part of the song that is both rhythmic and melodic in structure ā¦ many songs have iconic riffs that are instantly recognisable
examples:
The Beatles - Day Tripper
Led Zeppelin - Black Dog
Lick ā¦
a short phrase, mostly played as a lead guitar part within a solo or as a small fill between vocal lines ā¦ these can be the foundation of solos or strung together to create solos, they are often mixed with improvised sections and some players have ātrademark licksā that they use as a default home base to start from and return to when improvising
examples:
hard to give because they are not riffs ā¦ see Justinās minor pentatonic lessons for five licks in each of the five scale patterns
Most riffs and licks are both played using notes from scales so that may explain some of the confusion.
You just couldnāt let that slide could you? 
But both a lick and a riff can. Slide that isā¦
You can remember the distinction if you think like a dog. Riff riff riffing is generally rhythmical and can go on for an extended period of time, whereas licking stops once the novelty or flavor wears off 
my immediate reaction is that you can construct a riff from one or more licks strung together and/or repeated, but not vice-versa
Youāre doing well David, my kids think I never listen to anything recorded after 1985 (coincidentally the year my eldest ākidā was born!)