Sirventes usually (possibly, always) took the form of parodies, borrowing the melody, metrical structure and often even the rhymes of a well-known piece to address a controversial subject, often a current event.
The original piece was usually a canso, but there are sirventes written as contrafacta of (at least) sestinas and pastorelas.
The first author known to have written a sirventes is Cercamon, the name of the genre was first mentioned by Marcoat, and the most famous practitioner of it was Bertran de Born; Peire de Vic was also known for his sirventes, but only one has survived to this day.
Most major troubadours have written at least one sirventes; well-known examples include: The sirventes, called sirventesch in early Catalan, was imported into that language in the fourteenth century, and it developed into a unique didactic/moralistic type.
Dalfin je us voill desrenier by Richard I of England is a notable example of a sirventes written in Old French.