Some pistons control stops -- General controls the whole organ; divisional pistons only one division. Usually these are numbered.
Tutti/SFZ/Full Organ is a reversible that turns all stops on -- and off again. A reversible is press once to turn on; press again to turn off. Sw/Gt, Gt/Ped and the like are reversibles.
PP to FF are preset pistons for a general setting of particular volume. Usually found on theater organs.
< and > are stepper pistons that change combinations in a certain order. Keep pressing the > piston to move to the next combination in the sequence.
With Hauptwerk, any piston can be any configuration. It all depends on what you want it to be. There are Generals, Scoped (which can act like divisional that only affect a designated set of stops), reversibles, and stepper. Same with toe studs, which are just thumb pistons for the feet.
If you're just getting started, you MUST get Kenneth Spencer's book mentioned in the top section of this forum. It explains TONS about such matters.