You really have to get your terms right to fully understand it. A head *is* the amplifier. The cab/ stack is just the speakers that the amplifier plays through.
Your cab has to be *at least* the same wattage as your head, otherwise things will start blowing up if you start to crank it like Pete says. Some people like to match up the wattage of their head and cab because the speakers will start to naturally distort earlier, i.e. crunchier overdrive. I myself have a 50w head with a 160w cab because I like to have as much clean headroom as possible, and having speakers built to withstand much more power than I can put into them gives me that. It's really a personal preference thing, as long as your cab is at least as powerful as your head then it's entirely up to you.
