Different combinations of musicians are given different names, based mostly on personnel, instrumentation, and the style of music played.
An orchestra, traditionally, is made up of the following: strings (violins, violas, cellos, bass), brass (trumpets, trombones, french horns), woodwinds (clarinet, oboe, bassoon, flutes, piccolos), and percussion (snare drum, bass drum, cymbals, misc. percussion instruments). Although there are exceptions, an orchestra plays mostly symphonic music (Mozart, Bach, Beethoven, etc.) and most often performs in a concert setting. Most orchestras are seated the same way with the violins and violas on the conductor's left, cellos to his right, woodwinds behind the strings, brass in back to the conductor's right, and percussion, back center. Its quite common that a piano is included to the conductor's immediate left. A soloist usually takes this position as well. The average size of an orchestra is 75 to 100 players. Smaller groups of 50 or less are often called chamber orchestras.
The word orchestra is sometimes used in a less informal way such as the Ukulele Orchestra of Great Britain. The Boston Pops orchestra is known for playing popular songs of the day.
A concert band, unlike an orchestra, has no stringed instruments such as violin or cello. And where an orchestra usually has three trumpets, a band can have as many as twelve (as well as that many trombones and clarinets). Although concert band music can range from symphonic to popular to Jazz, many bands are known for playing marches such as The Stars and Stripes Forever and The Washington Post March. Some bands play both in a concert setting and perform as a marching band (such as in parades or as part of the entertainment at a football game).
The word band, of course, can also be used to describe a rock band, Dixieland band, or hip hop band. Personnel and instrumentation varies widely in these kind of groups as well as does the music being played.
A choir is made up entirely of singers (and often a piano accompanist) and most often takes the form of male and female vocalists divided into five voices (vocal ranges): soprano and alto (women) and tenor, baritone, and bass (men). Music written for choirs utilizes the five voices to create the parts that might otherwise be played by musical instruments. Choirs can be all male or female and there are many specialty choirs such as singers that perform only certain styles of music. Jazz choirs are quite common as are barbershop quartets.
An ensemble is a "catch all" phrase for a group of musicians. The term is mostly used to describe string ensembles. The term is sometimes used when describing a group of singers (e.g., a vocal ensemble). Typically, an ensemble contains four to twenty members. It is derived from the word "assembly."
The word combo (from the word "combination") is sometimes used when describing an ensemble, but for the most part, it means a four-or five-piece group, typically with guitar, bass and drums. You'll see the term most often when describing jazz musicians (i.e., a jazz combo).
