Basically, Elmore James popularized the electric guitar as an instrument for blues music.
He normally played clean, without the distortion that would come to herald the sound of Rock and Roll, but on tracks like Country Boogie (among many others) Elmore demonstrated that he could rock with the best of them (and years before many of the rest of them started rocking.)
He's one of those often overlooked faces in the history of blues and rock and roll, and that's a damn shame.