Prog rock, short for “progressive rock” is a form of rock music that decided to break out of the typical 3-chord, 3-minute song forms that dominated the early era of rock. Basically, it was an attempt to elevate the rock artform into something more creative, intellectual, expansive, complicated. Among other signatures of the genre, there was an emphasis placed on instrumental (technical) virtuosity; longer, more involved song structures that went beyond the verse-chorus blueprint of popular rock; an expanded harmonic vocabulary; rhythmic complexity (no longer bound to 4/4 or 3/4 of most popular music); and lyrical themes beyond sex, drugs, and rock & roll. Prog rock was, in a way, trying to bring the sort of respect that classical music got to rock, by creating a more complicated and more virtuosic art form. It has nothing to do with the presence or absence of keyboards, but they seem to be a favored instrument of progressive rock bands.