there are two parts of an amp.
the preamp, brings your signal up to line level, this is also where your eq is. it sends it through the phase inverter and into the power amp which which amplifies alot. this is where most of the volume comes from
a head is composed of 2 items: a power amp and a preamp.

first is the preamp, which boosts/cuts frequency tones (such as the low/mid/high/gain pots you may see). preamp also add effects commonly overdrive. some preamp have more effects, but you can always add a multi-effect (see below).

next would be the multi-effect. this is where many effects are added. such as tremolo, chorus, overdrive, reverb, and every other awesome effect to play around with. usually they use some sort of midi interface, but i wont go in depth now.

lastly (before the speaker) is the poweramp. the poweramp is only responsible for 1 task: boost the small audio signal that comes out from the previous component and make it louder. much louder, so that you can actually hear the sound.

as i said before, the head includes all there components built into one unit, but all alone these units can be mounted in a racketmount. that way you can have them all together, yet have much more flexibility when upgrading your gear.