Watch the Level 3: Jazz Blues Soloing online guitar lesson by David Hamburger from Take 5: Jazz Blues Soloing

There's a lot more nuance to it than this, but essentially hard bop was a distillation and reframing of bebop which took place between roughly 1955 and 1965. As described in David Rosenthal's excellent book Hard Bop and elsewhere, musicians like Art Blakey and Horace Silver made a conscious attempt to integrate the bebop language developed by Charlie Parker and Dizzy Gillespie with more blues-based grooves like the shuffle and more song-like, memorable melodies. Sounds perfect, right?