It's a traditional song form that many standards are written in, and usually is comprised of 32 measures, each section being 8 measures long.
Play the first section - "A"
Play the "A" section again
Play the "B" section (usually the bridge)
Replay the "A" section.
In Jazz terminoligy each AABA is a "chorus" the first play through uses a written arrangement with the tunes recognizable melody, and is called the "head". The following choruses maintain the tunes harmonic structure and are used as a basis for solos by the various musicians in the group. Each solo is commonly 1 chorus long (though frequently two or more musicians will "trade" solo sections of 2 or 4 measures each during the chorus). Lastly the head is played to end the tune and bring it back to recognizable territory.
When vocals are used the "A" section is most commonly used for a verse, and the "B" Or bridge section is either a bridge or chorus (different term).
Other commonly used song forms are, AABC, AAB, and 12 bar blues.
...............Jim
