I think maybe you're reading posts with people talking about having live rock "rubble" in their sump. Depending on how you buy your live rock, you might end up with smaller, unusable chunks that just don't look good in your main tank. People throw those chunks into their sump to help add more area for the beneficial bacteria to colonize. The main amount of live rock, if not all of it, goes in the main tank. It may look ugly when you get it, but after a while it'll take on a whole life of its own.
You don't have to put everything in a sump if you don't have room. Just in case you haven't run across it yet, the other option you've got is to use "hang on back" or HOB
equipment. There are many good skimmers out there that can just hang on the back of the tank. That goes for power or cannister filters also. Me personally... I've got a hang on back skimmer, as well as a hang on back canister filter that work just fine for me. I'd LOVE to have put all that stuff in a sump out of sight and my ears, but space constraint for me just didn't allow a sump at this point. If you have the room... go for it.
You're correct... there's TONS of stuff to learn, and relearn about this hobby. If you haven't found mention of it yet, a good starting point is "The New Marine Aquarium" by Michael Paletta. It's a great starting book that's a pretty easy read. After you're done with that one, the big kahuna of books is "The Conscientous Marine Aquarist" by Bob Fenner. It's a little more difficult of a read, solely because of the amount of information packed into.
Welcome, and remember to have fun!