The way I usually define prog and progressive as it relates to music:
Prog - a style of rock music that incorporates within individual songs, a number of time changes, technical playing and a variety of instrumentation in a non-standard format (i.e. different than the normal intro, verse, chorus, verse, bridge, solo, chorus, ending format).
Progressive - A rock artist that incorporates a variety of music styles into its core sound from one album to the next.
To me, that's about as best you can define the two without going ridiculously overboard.