The word Motet is used to mean: a piece of music for a church service sung by a choir without instrumental accompaniment. The words are usually in Latin. If the words are written in English for the Anglican church it is called an anthem.
Motets have been written since the Middle Ages. Medieval motets often had very complicated rhythms. The tenor often had the tune, which might be a folk tune, and two voices would put quite complicated accompaniments on top. The most famous medieval composer who wrote motets was Guillaume de Machaut (1300-1377)
In the Renaissance motets were usually polyphonic. That means that the different voices (soprano, alto, tenor, bass) would be singing different parts of the melody at the same time, making it very beautiful but quite complicated to listen to. Many composers of this time wrote motets. The most famous was Giovanni da Palestrina (1525-1594).
In the Baroque period one of the most famous composers was Heinrich Schütz(1585-1672) who wrote motets which he called “Cantiones Sacrae” (meaning: “sacred songs”). Johann Sebastian Bach (1685-1750) wrote six wonderful motets with German texts. Some people think that Bach may have used a few instruments to accompany these motets. In three of them the choir divide into eight parts.