Yeah, I concur for the most part with Gilles' definitions, but the fact is, the details of each can vary from player to player. There isn't a guitarists' universal lexicon or anything, just a generalised consensus on the broad meaning of terms like that.

A "song" usually refers to a single piece of music that is based around the lyrical content, as opposed to an "instrumental" which is based around... well, the instruments (!), or a "piece" which can refer to either. The musical devices that make up the song are collectively called the "tune", and within the tune, you have "riffs", which are devices that feature repeatedly and/or prominently. As well as the tune, you have the beat, which is the pulse of the music in relation to the time signature. In between the beat and the tune, you have the rhythm, which forms a base for the tune by relating to the beat.

In summary: Song =
#Tune - riffs
- melody
#Rhythm
#Beat

Hope that helps some - and sorry for the mammoth post!