Bebop really started in the late thirties with such luminaries as Charlie Parker and Dizzy Gillespie. Essentially what they were doing was taking standard tunes, extracting the chord sequences and putting their own tunes on top. Another major difference was to use the higher intervals, 9ths, 11ths and 13ths. Also instead of using 8th notes, the trend was to double time into 16th notes. As DMC rightly pointed out, flatted fifths also became popular, particularly with Thelonius Monk. It was musicians' music in the sense that you couldn't understand it unless you had been there. I stopped playing jazz in the fifties because I didn't have the musical ability to play the new stuff at the time (I'm a little better these days). The bebop era lasted into the late fifties but with the death of Bird the life went out of it somewhat. Miles Davis put an end to it with his introduction of "cool" jazz and Bill Evans changed piano playing for ever. Cannonball Adderley and John Coltrane continued with it to some extent but when they played with Miles the trend shifted to modal styles and long solos. One quote from Miles - "We had a session and I only got to play two choruses - Coltrane and Adderley took fifty choruses each"

Bryan