The basic components you need for a sump:
Overflow: could be as simple as a piece of PVC pipe in your main fish tank that lets the water flow from the main tank to the sump. You can also buy overflow boxes where the water goes against gravity to get out of your DT for a second(display tank/main tank), and flow down to your sump.
Drain line: the pipe that is hooked up to the overflow, drains the DT to the sump
Sump: any container of choice that holds water, bucket, trash can, used aquarium, anything.
Return pump: the pump that takes the water that went into the sump and puts it back into the DT.
Return line: the pipe that is connected to the return pump that allows the water to return back to your DT.
That's it. In the simplest form that is a sump. Like I said the goal is to expand water volume, most people just use it to their advantage to hide equipment underneath the stand (heaters, pumps, dosers, etc) and they will also put baffles (walls) in the sump to create chambers for different things like refugiums or places for the protein skimmer to sit. A sump is great because if you have a small tank, like your 20 gallon, you could stick another 20 gallon tank underneath your DT and double your water volume. The more water volume the easier and more stable the system tends to be. Go to the link and read up!