Is the tank going to be fresh or salt water? In both, you can put your heaters, filter media, etc. in the sump, and it can provide quite a bit of flow to the tank.
In freshwater, this is just a place to put all of your filter media, your heater, etc., I've never really looked into freshwater sumps, so I don't really know about them either.
In saltwater, you have lots of equipment that you can put in a sump, all of your media, and a refugium is a great thing to have, as it helps filter out nitrates, and it helps to grow copepods, which many fish will need to eat.
How it works is that there is either a hole in the side or the bottom of the tank, and water falls down an overflow weir, into the tube and down to the sump, down below. Then, you have glass baffles separating different sections, for different pieces of equipment, and media. In the final compartment, there is a big pump that pumps the water back up to the main tank.
Just a warning if you're drilling the tank yourself: make 100% sure that the glass on the tank is NOT tempered glass. I've had experience drilling tempered glass, and it doesn't work, the whole piece of glass cracks into a million little pieces. I was pretty bummed when I found out the glass on my brand new 90 gallon tank was tempered the hard way.