This is how I explain it in super simple terms, although I'm sure someone like @agedhorse would have a much better way to give to give you much better information in more correct technical terms:
- A "head" is an amplifier, although usually it's actually both a pre-amplifier and an amplifier.
The pre-amp is what makes the tone (bass, mid, treble, etc.) and the amplifier is what makes the volume.
- A "cabinet" is a box with speakers in it.
- A "combo" is where the amp and the cabinet are in the same box.
- Having separate components gives you more (though not necessarily better) choices.