Ternary form is a three part structure. The first and third parts are identical, or very nearly identical, while the second part is sharply contrasting. For this reason, ternary form is often represented as ABA. The contrasting second section is often known as a trio.
At least in pieces written before the 19th century, the first section of a piece in ternary form does not usually change key, but ends in the same key as it began. The middle section will generally be in a different key, often the dominant of the first section (a perfect fifth above). It usually also has a contrasting character; in a march, for example, the highly rhythmic and strident character of the march itself, is usually contrasted with a more lyrical and flowing trio. Less commonly, the trio may also be in a different time signature (3/4 as opposed to the 2/4 of the march, for example).
As well as marches, ternary form is often found in baroque opera arias (the so-called da capo aria) and in many dance forms, such as the sort of polkas written by the Strauss family. It is also the form used in the minuet (or scherzo) and trio, which in the classical music era was usually the third movement of symphonies, string quartets, sonatas and similar works.
A distinction is sometimes made between compound ternary form - in which each large part of the form is itself divided in a way to suggest ternary or binary form (giving, for example, an overall scheme of ABACDCABA) - and simple ternary form, in which each large part of the form has no particular structure itself. Da capo arias are usually in simple ternary form, minuets (or scherzos) and trios are normally compound.