I am a bit confused about the differences in the composition of metal and progressive metal. Are there major differences in their orientation of sound, or it is just the difference of incorporating more instruments in metal.
Labels are only of limited usefulness. However, it can be observed metal is ultimately based on the blues and rock and roll. Metal songs are usually closer to the song structures, rhythms and meters of blues and rock--they're just played much louder and harder.
Progressive metal incorporates elements of progressive rock and classical music, especially frequent changes in time signature and meter, the use of odd-time signatures (like 5/4 and 7/8) changes in keys within songs, more complicated chord progressions (expressed through arpeggios because strumming complex chords doesn't work with all that distortion). Progressive metal compositions usually involve longer pieces and less reliance on song form (verse, chorus, bridge, etc.). Progressive metal is also more open to using instruments other than drums, bass and guitar.
I agree with Wheat Williams.
the problem is that the metal genre has (too many) subgenres. Many metalbands are categorizable in several subgenres.
Progressive metal is a blend from the early '80 progrock bands (Gentle Giant, Yes, King Crimson) and the more modern metal influences. It seems that where other metal subgenres like metalcore, deathmetal, folkmetal and Doommetal resemble in sound and structure, progressive metal bands may differ extremely. (Dream Theater, Opeth, Porcupine Tree Pineapple Thief, Novembre). Bands that found a unique sound are often quickly identified as "progressive", while there's no 'core property' which identifies the subgenre as such. This also makes it difficult to say "I like progressive metal!".