In popular music, a bridge is a contrasting section which also prepares for the return of the original material section. The bridge may be the middle-eight in a thirty-two-bar form (the B in AABA), or it may be used more loosely in verse-chorus form, or, in a compound AABA form, used as a contrast to a full AABA section, as in "Every Breath You Take". Very commonly the "bridge" is in a contrasting key to the original melody. More often than not, the "bridge" is a perfect 4th higher. For examples, see Richard Rodgers' "Mountain Greenery" and Antonio Carlos Jobim's "Meditation" just to name two.
Lyrically, the bridge is typically used to pause and reflect on the earlier portions of the song or to prepare the listener for the climax.
The term may also be used to refer to the section between the verse and the chorus. Although this is more commonly referred to as the pre-chorus, it is not completely incorrect, as often the transition between the two themes of a sonata form in classical music is similarly referred to as a bridge. A more formal way of describing this transition between two themes (in classical music structures) is by referring to it as the "transition theme".
(Courtesy of Wikipedia)