In a saltwater sump, as Travis said, you have to have I think 3 chambers. A settling chamber where you may see filter socks, a chamber with steady water level where the protein skimmer goes, and then a final chamber where the pump goes. There can also be chambers for a refugium where they grow "pods", small animals that small fish eat in the display tank. If the pods were hatched in the tank, the fish would eat 'em all and they'd never grow to a self-sustaining population--so they try to hatch them in the sump and let them migrate to the display tank.
A freshwater tank doesn't need any of that. However, some do still use pads or bio-balls for filtration in FW sumps, and that requires chambers I think. I don't use any chambers at all in my tank. I just have one big open sump.
that being said, you could certainly use a manufactured sump designed for marine tanks to filter a FW tank. You'll probably just have more chambers than you need. But it'll still work just fine.