A Duet involves just two performers, for example, piano and voice, or piano and violin. The word is an abbreviation of Italian duetto, meaning 'a little two' from due = two. Another term often used is duo
In some cases, there may actually be 3 performers, for example a work for two singers accompanied by a piano might be described as a duet, emphasizing the two singers alone. There are many examples of duets in opera where two singers are accompanied by the orchestra.
Piano duets for one piano, four hands, are very popular with amateur pianists, as this is an excellent way for people who do not often get an opportunity to play in ensembles, to experience making music with other people. Many famous composers have written works especially for piano duet, and also made arrangements of their own orchestral works as a piano duet.