In the most general sense, the term "brass band" is applied to a large ensemble that contains a significant number of brass instruments. For example, military [wind] bands have sometimes been referred to as "brass" bands, presumably to distinguish them from military pipe bands. Since military bands commonly comprise brass (trumpets/cornets, horns in F, baritone horns, euphoniums, trombones and tubas) and woodwinds (piccolos, flutes, oboes, bassoons, clarinets, saxophones), "brass band" is a misnomer in such cases.
See the Wikipedia article on brass bands.
The most rigorous definition applies to the "British brass band", comprising a fixed set of instrumentation, in specific numbers, totalling 25-28 members. This rigor is necessary because, in the United Kingdom and Australia (and to a minor degree, the United States and Canada), brass bands engage in judged competitions between bands.
The British brass band is made up of:
1 - Eb Soprano Cornet
9 - Bb Cornets (4 Solo, 1 Repiano, 2 2nd, 2 3rd)
1 - Bb Flugelhorn
3 - Eb Horns (Solo, 1st, and 2nd)
2 - Bb Baritone Horns (1st and 2nd)
2 - Bb Euphoniums (1st and 2nd)
2 - Bb Trombones
1 - Bb Bass Trombone
2 - Eb Basses
2 - BBb Basses
n - Percussion (0-3, as needed)
The Internet Bandsman's Everything Within reference site provides a more technical and detailed explanation of the instrumentation, under the heading "What is a Brass Band?".
There is a substantial body of music ("the literature") written, arranged, or transcribed for British brass band. In the early days (say, 150 years ago), classical and popular pieces of the day were transcribed and arranged for British brass band. Today, while much current music (e.g., from musical theatre, movies, popular music, etc.) continues to be re-arranged for brass band, there are many pieces written specifically for these ensembles. These original pieces are often written under commission, or as contesting "test" pieces.
A "British-style" brass band can be defined as an ensemble that plays the British brass band literature, but exercises some flexibility in the instrumentation and numbers. In North America, Eb horns (also known as "Tenor horns" or "Alto horns") are uncommon: instruments and people to play them can be difficult to find. Hence, a Horn in F might serve as a substitute. Similarly, Trumpets are more commonly found than Cornets, and you may see some Trumpets in the "Cornet" section. British-style bands usually do not compete, so the number of players per part may vary from the rigorous British brass band specifications. For example, Salvation Army bands frequently follow the instrumentation of British brass bands, but may vary the numbers.
Beyond "British" and "British-style" brass bands, Americans also use the term "brass band" in a very loose way to describe medium- to large-sized ensembles comprising mostly brass instruments. Often, these "brass bands" recreate ensembles from the last half of the 19th century in the United States. To illustrate, take a look at the Great American Brass Band Festival Web site.
Updated 2004 Oct 10, 14:40 EST/EDT