Sanchan, I know this is not standardized by any means, but in the guitar world, a riff if usually a short pattern, often on the bass strings, which is actually the "basis" of the tune, whereas a lick is a pattern of notes and techniques such as bends, slides, etc., which are generally played in an ad-lib, or soloing situation. An example of a riff might be the opening notes of "Day Tripper" by the Beatles, or the power chord intro for "Iron Man" by Black Sabbath. Bill Monroe's intro for "Muleskinner Blues" would be considered a lick.
"I thought I knew a lot about music. Then you start digging and the deeper you go, the more there is."~John Mellencamp
"Theory only seems like rocket science when you don't know it. Once you understand it, it's more like plumbing!"~John McGann
"IT'S T-R-E-M-O-L-O, dangit!!"~Me