they are terms to describe anything from a part of a song, to technique

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a lick is any segment of a song, they are often very defined and independent within themselves and can even the whole song, depends on who you're talking to, same for a run, though those are usually less defined and could be considered as parts of a lick, both of the aboive could also mean excercises or such
a riff is the main rhythm track or motif to a song, say, the main part of 'Back in Black' which comes in after the cymbal countdown and lasts through most of the song
a trill is a technique in which you repeatedly and quickly play one note for a certain duration of time in a way that they overlap to create the feeling of a single, almost vibrating note of regular/basic length, such as that used in the main portions/ riffs for misirlou (pulp fiction theme/opening song) or knights of cydonia
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Quote by Mechanix

We play guitar.... we're automatically on top of the world.

I tend to think of riff and lick as being rather synonymous. Both of them usually refer to a musical phrase or some other section of a song.

Trills are associated with rapidly switching between notes, basically rapid hammer-ons and pull-offs.

When I think of runs, I usually think of something technical or some hand coordination exercise.

Here's the thing, I don't really think there any clear definitions of lick, riff, and maybe even run. They're all pretty much relative terms rather than absolute, and depending on who you're talking to they may mean different things....
what is a search function?
what is google?

basically if you go to the lessons section, i think these are explained there. also, try to use google.
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lets go into that broom closet and find out?
Quote by Gabel
You are EXTREMELY WRONG! I have played it. I own an 18W and it would be an awful stereo amp, it's way too bright, breaks up too easily and so on. Secondly, why would a guitar store sell an hifi amp.