Sumps and fuges are really two separate things, although they can often be fit into the same clear box.
A sump is a good place to put your skimmer, heater, bags of carbon, and whatever devices you don't want in the main tank.
The fuge is for growing stuff, like macroalgae, amphipods, mysis shrimp and the like, to export nutrients and produce planktonic food. If you put it on a reversed lighting cycle (light when the main thank is dark), it will also help stabilize tank pH.
A 10 gallon tank would make a nice fuge, and might work as a sump, but is too small for both. Also, if you are thinking about putting a skimmer in there, you will need room for the skimmer (most online place provide dimensions), some wiggle room, and some space for baffles to keep bubbles from getting to the return pump.
The link that FL Knifemaker posted should be really helpful.
