Hard bop is one of those "know it when I hear it" kind of things for me. I don't know a good definition for it. I don't know if there even is one. If there is, I haven't seen it, yet. I think it would include bebop phrasing and flourishes (see Benny Golson's soloing on the Blue Note Art Blakey and the Jazz Messengers classic "Moanin'") but feel funkier, more syncopated, with harder edges and heavier aurally, and with more prominent swing and blues elements, among other things. If a good bebop performance can amaze and delight, a good hard bop performance can awe and excite.
Like "Moanin'" the title track on the Blakey and Messengers CD, of which you get two standout versions on recent issues of the disk. Either version of the song, written by pianist Bobby Timmons and refined in this case by Golson, has to be considered a seminal jazz performance and prime example of what hard bop is. "Moanin'" probably explains it better than hanging words on it can.