well ORIGINALLY progressive rock/metal was meant to be a style that waswhere people tried new things and experimented and just tried new stuff, for better or worse (although always better in the musicans mind hehe, not always the fan) And the sound wasn't so easily categorized. Now it has almost started to develop a familiar sound and the tr00 progressive fans are getting bitter heheh...
Prog metal is most commonly associated with the Dream Theater sound (odd time sigs, a certain degree of technicality, etc.) band like Dali's Dilemma, Threshold, Andromeda, etc.
But then there are the few bands who do stuff that is still "accepted" by the tr00 prog fans, bands like Opeth, Pain Of Salvation, Gordian Knot, and Dream Theater themselves (even though MANY have copied their sound, they came up with it). That is prog metal.
Prog rock is something a little different. It is (obviously) not as heavy and came about in the 70's, with bands like Rush,Yes, King Crimson, ELP and such. Some of the newer bands in that genre are Anglagard, Spock's Beard, and The Flower Kings (yeah, that IS a bad name hehe) Basically it is quite similar to prog Metal, only softer and often a bit more experimental with sounds and structure.
Anyway hope that helps.