A "band" is loosely defined as a small to medium-size ensemble of musicians. Exactly what constitutes a "band" depends on the exact type of "band", and the term without any qualifiers has referred to different ensembles over the years.
Originally, the term was synonymous with "brass band" or "military band", which were a component of military forces up until shortly before World War I, charged with keeping the unit entertained and on-pace while marching, especially during parade maneuvers. The key distinction from the modern military band and an "orchestra" is the lack of a string section, although in the military of the 1700s and 1800s, violinists were relatively common if not always official members of the band.
Today, military-style bands are more often seen at the high school and college level as "marching bands", with a difference made between true military bands like Texas A&M's marching band, focusing on accuracy of basic formation maneuvers, to "show bands" which use common woodwind and brass instruments in more complex formations, to "drum and bugle corps" which are restricted to specialized two-valve "bugles" of various sizes, and march in competitions with formations and programs similar to show bands. Actual military music groups run the gamut from jazz ensembles to full symphony orchestras, and put on concerts for deployed soldiers and at special functions.
In the 1920s and 1930s, the term "band" was most commonly associated with the "jazz band", a general evolution of the brass band into 20th century popular music styles, typically consisting of a horn section (usually clarinet, saxophone, trumpet and trombone), an upright bassist, kit drummer, and myriad additional instruments as may be called for, including guitar, banjo, violin, flute, keyboards, and even tuba (in "New Orleans-style" jazz).
Today, the term "band" is most commonly associated with a group performing various pop music genres, typically composed of a subset or other variation of the following; a kit drummer, bass guitarist, keyboardist, rhythm guitarist, lead guitarist and lead singer, with some genres like funk, ska and Motown also calling for backup singers, a horn section (trumpet, trombone and/or saxophone), and (in more modern times) a scratch DJ.