A Brass Band is, typically, a wind ensemble consisting entirely of brass instruments, though usually with the addition of percussion. There are usually up to about 24 players and the instruments used consist of cornets, horns of various registers, tenor and bass trombones, variously sized tubas - ranging from tenor (euphoniums) to bass - and the percussion – usually bass and snare drums and cymbals.
The form became popular in the early to mid 19th century in Northern England, nearly always attached to a factory, colliery or foundry, and spread throughout the English speaking world. In 1900 there were an estimated 5000 bands in England and Wales alone competing in the various competitions. The form, with small variations) and usually with fewer personnel, spread to to the Salvation Army.
At first there was a paucity of quality band music , however, in the early 20th century, serious composers such as Edward Elgar, Ralph Vaughan Williams, Gustav Holst and John Ireland began writing or arranging music for brass band.
Military bands are usually “augmented” brass bands. Added to a core of brass instruments can be saxophones, clarinets, flutes, bassoons and others that, whilst blown, are not brass. Such are “authentic” renditions of George Frideric Handel's “Water Music” and “Music for the Royal Fireworks”. That enormous army of instruments led by “Seventy Six Trombones” and “Professor” Harold Hill down the main street of River City in “The Music Man” was a brass band augmented to within an inch of its life.
Brass Bands contests are still extremely popular and are the staple of the genre. These days, elite ensembles such as the “Grimesthorpe Colliery Band” and the “Black Dyke Mills Band” with their roots deeply imbedded in the grim soil of working class northern England tour the world, playing in classical concert halls and outdoor venues and enjoy multi-million dollar recording contracts.
Reference:"Oxford Companion to Music"