What "pump" leaks in a 700R4

[Skunksmash]

I had my trans fixed for about $800 a few years ago when I got the truck. It was leaking and I'm fairly ignorant on the guts of an automatic trans. The guy wanted to do a full rebuild for $1500, but I said that I just couldn't afford that. So he said that he could just fix the leak and do a few other minor things including the rear main seal, for about $800 after everything.

I didn't know enough about the auto trans and still don't even now, to understand what "pump" was causing the trans to leak. Anyone know? Something I've always been curious about. Cause I really don't like not knowing what a mechanic is talking about when I have to have something worked on that I can't just do myself.