Why amps, pre-amps, integrated amps???

OK, having thusfar asked questions on this forum that have exposed me to the odd raised eyebrow and snicker for my gross audio ignorance, I shall go farther still, and venture to ask: What, exactly, are amps, pre-amps, and integrated amps??. More to the point, what, exactly, is their purpose; what do they do? And why do pre-amps and amps still exist comfortably in the audio market when you can get them combined as an integrated amp?? I just don't get it. Would much appreciate your learned revelations - after, of course, you've finished with your hoots, knee-slaps, and cat-calls.