they are two type of coilovers. I AM DISCUSSING HEIGHT ONLY, NOT DAMPING.
Single sleeve type and dual sleeve type.
Single sleeve type are like GCs, tein basic, ss, apexi. They have a threaded sleeve with a ring (seat that spring sits on) that has another ring to lock it in place. this ring can be threaded up or down to raise or lower height. it is done by hand using coilover wrenches. this type uses progressive rate springs (which are longer and more comfortable).
Dual sleeve type are tein flex, omni, ksport, d2 etc. Dual sleeve type incorporate the single sleeve coilover BUT also have an adjustable shock body. the shock is threaded and threads into a seperate shock body. the shock can be threaded up or down into this seperate shock body. this is considered far superior to single sleeve in terms of performance. This type also uses very linear springs (and as a result springs are shorter and a lot harsher). These are also adjusted by hand and require wrenches.