Vince-
Good question. PC stands for compact flourecent and VHO stands for Very High Output. NO stands for Normal Output and MH stands for Metal Halide. LR stands for live rock and LFS stand for Local Fish Store. LS is Live Sand and DSB is Deep Sand Bed. RO/DI is Reverse Osmosis De-Ionization. GPH is Gallons Per Hour. WOW...that just stands for WOW.
AS far as the difference between them, this is serious can of worms you are opening here. do a search on VHO vs. PC and you will learn a ton.
much luck
emile