jellybelly said: I was also confused when I was young: I blame marshall for putting speaker grille cloth on its heads
There are, contrary to what people who I'm guessing have never owned a stack would have you believe, several important diferences:
-with head-and-cab the tubes and reverb (if it has it) will not rattle at high volume: this can be an annoying issue if anyone has experience with a combo at gigging volumes
-cabs are generally speaking, closed-back whilst combos are open-back: closed back is more focused, has MUCH better bass response and tightness and better projection whilst open back is better for clean sounds to 'bloom' I guess the best word to use would be. If you're into metal then you will be dissappointed with an open back, same for heavy/tight rock, but everything else an open back will do just as well
-also tube amps can be very heavy so often easier to carry two parts seperately
-as mustang-pete said, you can try an amp with many speaker configurations, and also just take a head to practice and borrow a house cab: most places I've been generally have a few beat up marshall 1960As knocking about.
Get a combo until you know exactly what kind of sound you want; it'll be cheaper and more compact