A musical ensemble is, by definition, a group of three or more musicians who gather to perform music.

There are several denominations of ensembles according with their size and composition.

The terms duet, trio, quartet, quintet, sextet, septet, octet, and nonet are used to describe groups of two, three, four, five, six, seven, eight, and nine musicians, respectively. In classical music, these arrangements are commonly referred to as chamber music. A common quartet is the string quartet, composed of two violins, a viola and a violoncello. The most usual string quintet is similar to the string quartet, but with the viola duplicated. In some cases, though, it is the violoncello that is duplicated. See: String trio, String sextet, string . A piano quintet is usually a string quartet plus a piano. Another fairly common grouping in classical music is the wind quintet, usually consisting of flute, oboe, clarinet, bassoon and horn.

In jazz, a fairly standard trio line up would consist of a rhythm section of piano, bass and drums. A quartet would typically add a horn (the generic jazz name for saxophones and trumpets) while larger ensembles would add further instruments. The actual lineup of jazz ensembles can vary quite considerably though.

A group with more instruments is usually called an orchestra. A small orchestra is called a chamber orchestra. A symphony orchestra is a large body of several tens and often more that a hundred musicians, divided in groups of instruments: violins (I and II), violas, violoncellos, basses, woodwinds, brass, percussion, and sometimes more.There is a difference between Symphonic, Philharmonic, and similarly titled orchestras. These are names used to distinguish the nature of the musicians' job, and symphonic refers to professional musicians who work in the orchestra as a primary job, whereas philharmonic refers only to musicians who volunteer their time to the orchestra, and do not get paid for their participation in it. The difference between the terms is rooted in the Greek prefix philos (friend), and as applied in this context, it means friends of the orchestra. A Sinfonietta usually denotes a somewhat smaller orchestra (though still not a chamber orchestra), and the terms concert or pops orchestra usually mean an orchestra concentrating mainly on the light classical and more popular repertoire.

In the 1900's, the Wind Symphony or Wind Ensemble became popular, especially in academic circles. A wind ensemble consists entirely of wind instruments and percussion instruments, but may also include stringed bass. Schools from elementary level onward often have some version of a wind ensemble, often known as a concert band.

A string orchestra has only strings, i.e., violins, violas, violoncellos and basses.

A choir is a group of voices. Sometimes the group of similar instruments in an orchestra are referred to as a choir. For example, the woodwind instruments of a symphony orchestra could be called the woodwind choir.

A group that plays popular music is usually called a band.

Other Western musical ensembles

• Jug band
• Mexican Mariachi groups typically consist of:
  • at least two violins
  • two trumpets
  • one Spanish guitar
  • one vihuela (a high-pitched, five-string guitar)
  • one Guitarrón (a small-scaled acoustic bass).

Non-Western musical ensembles

• A gamelan is an ensemble of Indonesian origin (usually Balinese or Javanese). There are dozens of varieties of gamelan ensembles with musicians playing metallophones, drums, flutes, bamboo and wooden marimbas and gongs.
• The Steelpan created in Trinidad and Tobago are the core components of percussion ensembles called Steelbands that play Calypso music.