Alright,here is the lowdown(gansta)

all that expensive(for me)stuff is LOUD and sounds GOOD(depends).

there are amp combo's(what you have)

They come in many sizes and watts.Watts are a good way to judge how loud it can get or how much air it can push.100watts is the standard for big rigs,but 50watts and up should be fine for a gig.Speakers can range from a small 5" speaker to large 18" ones.The standard in combo's are 1x12 or 2x12(1 or 2 12" speakers).

heads are basically combo's without the speakers...you put them on a cab of your choice.Cabs come in tons of variations...a standard cabinet is 4x12 and it will rumble your chest(4 12" speakers).A speaker cable will go from the head to the cabinet.

thats really all you need to know...

if you want to know whats 'good' just look at the price...a good combo will be 400$ and up.A good head will be about the same or more.a good cab will run from 400$ and up as well.Anything under those prices will be reflected in the product(not that good deals are not out there).

The advantage of a head over a combo is that you can play the head through different cabs and the head may be cheaper.a combo's advantage is its portability.

there are speaker cables,instrument cables,microphone cables e.c.t.Get whatever you need.Quality can also be reflected in the price of the cable...your sound may be dulled by super cheap cables..there durability can suck too.

you get what you pay for...