A lead solo involves the lead guitarist playing for a full verse or chorus of the song (or longer), while the singer has a rest.
It may occur at the beginning or end of the song, but is more commonly found either half way or two-thirds of the way through the song. There are three basic levels of relationship between a lead solo and the melody line.
Level One - The lead solo consisting of an exact repeat of the melody (sometimes an octave higher for clarity). This occurs quite often in commercial "pop" songs where the lead break is relatively short.
Level Two - The lead solo being based on the melody, but having noticeable changes (e.g., in timing variations, extra notes added, and the use of techniques such as bends and slurs).
Level Three- The lead solo being completely different from the melody (this often applies to Rock songs, where the solo may last for a considerable length of time).