Rondo and its French part-equivalent rondeau are words that have been used in music in a number of ways, most often in reference to a musical form, but also to a character type that is distinct from the form.
Rondo form 
In rondo form, a principal theme (sometimes called the "refrain") alternates with one or more contrasting themes, generally called "episodes," but also occasionally referred to as "digressions" or "couplets." Possible patterns in the Classical period include: ABA, ABACA, or ABACADA. The number of themes can vary from piece to piece, and the recurring element is sometimes embellished and/or shortened in order to provide for variation.
The Baroque predecessor to the rondo was the ritornello. Ritornello form was used in the fast movements of baroque concertos. The entire orchestra (in Italian, tutti) plays the main ritornello theme, while soloists play the intervening episodes. While Rondo form is similar to ritornello form, it is different in that ritornello brings back the subject or main theme in fragments and in different keys, but the rondo brings back its theme complete and in the same key.
A common expansion of rondo form is to combine it with sonata form, to create the sonata rondo form. Here, the second theme acts in a similar way to the second theme group in sonata form by appearing first in a key other than the tonic and later being repeated in the tonic key. Unlike sonata form, thematic development does not need to occur except possibly in the coda.
Character type 
Rondo as a character-type (as distinct from the form) refers to music that is fast and vivacious – normally Allegro. Many classical rondos feature music of a popular or folk character. They include Mozart's Rondo in A minor K.511. Music that has been designated as "rondo" normally subscribes to both the form and character. On the other hand, there are many examples of slow and reflective works that are rondo in form but not in character.
A well-known operatic vocal genre of the late 18th century, referred to at that time by the same name but distinguished today in English and German writing by the differently accented term "rondò" is cast in two parts, slow-fast.
See also 
- Rondo and Ritornello Forms in Tonal Music
- Rondo form in traditional marches from Limoux' carnival
- Texts on Wikisource: