To answer your question, for systems with biological filtration, especially those with a protein skimmer, the sump is a smaller tank that the protein skimmer drains (overflows) into. In this setup, the display tank overflows at the top and drains into the protein skimmer. A return pump site inside the sump (submerged) pumps skimmer water back into the display tank. This is preferred by most aquarists and is very common here.
Your main saltwater options are FOWLR (Fish-Only With Live Rock) or reef (mostly corals, few fish). Live Rock is normally used for both. Live Sand may be used for a substrate as well, although some opt for "bare bottom". Both live rock and live sand are part of the overall biological filtration system. Reef tank is a significant step beyond FOWLR, with higher startup costs, higher running and maintenance costs, and more time.
Corals are the difference. They require higher-performance lighting and waterflow (powerheads), and they deplete Calcium and Alkalinity, which likely need to be supplemented.
You definitely want to think about what kind of aquarium you want to have, and what resources you're willing to commit to it.