Ternary form Ternary form is a three-part A–B–A structure in a piece of music. The first and third parts (A) are musically identical, or very nearly so, while the second part (B) contrasts sharply with it. The B section is often known as a trio. (From Wikipedia)
An example (Mozart Klavierkonzert 23, 2nd mov., brilliannt performance, imho, and sounds ein bißchen
Glucklich, which is great diesfalls :)
http://www.youtube.com/watch?v=N9aJOQG_CaM