A kind of piano dance music, influenced by blues and early jazz, that developed in black US subcultures sometime around 1920. Somewhat like a refined version of Barrelhouse Piano, boogie is not exactly intended to be played on a Steinway, but rather some scruffy, detuned, smoke-stinking wreck of a Baldwin rotting away in some dark place where prohibition-era gangsters hang out. The cliché instrument to play boogie on is of course not the mere trashy piano, but the honky tonk, a piano that is made to sound trashy on purpose.
Boogie-woogie is characterised by a 2/4 or 4/4 rhythm and repetitive bass patterns played by the left hand in hemiquavers, either straight or punctuated, but hammering along all the time, which can get very exhausting. Typical patterns include alternating fourths and fifths or walking-bass lines prowling up and down the bass side of the keyboard. The right hand plays a melody on a blues scale, making use of the usual tricks that have been used to approximate 'hot' intonation on the piano since the days of ragtime, i.e. lots of crushed notes and tremolo. Of course, there are always exceptions to the rule --
Boogie pieces are organised along a blues scheme. Actually, many boogies have "blues" somewhere in their title, such as the perhaps most famous boogie of all, "Honky Tonk Train Blues" by one of the greatest boogie players of all time, Meade 'Lux' Lewis. Unlike country blues, which goes around and around in a call and response scheme, boogie can't just repeat itself because there's no lyrics that can change. Thus, a boogie consists of several different parts one after another, which is clearly an influence by the European-inspired structure of ragtime. Boogie is not just blues done on piano.
With its simple structure and relentlessly driving rhythm, boogie was already a lot like rock'n'roll before rock'n'roll was invented. Boogie-woogie was actually one of most important influences for rock; the early piano rockers such as Fats Domino, Little Richard and Jerry Lee Lewis were boogie men.
Professional boogie pianists still exist today. Despite the "primitivity" of boogie as a musical style, some of them practise their boogie-ing up to ten hours a day. Whoa!
Boogie is also the way of choice to accompany old-style rock'n'roll on the piano, and, as many 1960s and 1970s rock outfits have proven, sounds good on a Wurly.