Well, I'm not RR, but I'll take a stab at it
Coilovers are height adjustable as opposed to just springs. You can get coilover "sleeves" such as Ground Control, Skunk 2, Weapon R, etc. that will require a set of struts to go with them. These don't provide a very nice ride quality.
True coilovers (Usually $900+) incorporate both the spring and strut together as a matched system. If you have the money, go for these. I'm eventually going to get the Apex'i WS true coilovers for my car, and they run around $850, which is a good deal.