Hey, I'm kinda really knew to the whole amp thing as I have just generally been playing an acoustic my entire guitar career except for my ibanez though a 15 watt practice marshall. I have been thinking about getting the JCM900 recently though but I'm a little lost about the whole thing. I don't know what a 'head' really does or if you need one to get an amp like the JCM900 or if that is just a 'cabinet' but i don't really know what that is either. i think a cabinet and an amp are two different things and you only need a head for a cabinet but I really don't know. If you could help me out with this that would be great. thanks.
if you look on your marshall practice amp it has all the knobs connected to the amp eg the volume controls etc a head is basically that but it is not connected to the actual speaker you have to buy the speakers seperatly.
Head= volume control, gain, bass, contour, treble etc. some have extra features like reverb delay and more.
a cab is basicly, just a box full o' speakers, most commonly 4 x 12 inch speakers.

The head is what you plug your guitar lead into and has all the volume control nobs and all that kinda fing, you have to have a cab to hear the sound from the head and equilly you need a head to send the sound to the cab.

The above is called a half stack, and will be EXTREMELY lOUD!!
Slightly off topic but, what's the difference between a half stack and what I suppose would be a full stack?
21st Century Digital Boy
Half stacks consists of one cabinet and a head, full stacks have two cabinets and a head.