Overflow is a filtration conponent prefered in saltwater that allows water feeding into the filter to come from the surface of the water. The best kind is completely contained inside the aquarium, and the water comes out a bulkhead fixture in the bottom. Less desireable is the "overflow box" that hangs over the side of the tank, and is more prone to failure. This works best with a wet/dry filter or a sump setup. A canister filter running in this config may suck air and stop working.
A versa top is a glass cover, with a small plastic strip in the back that you can trim holes in for accessories. I use one in winter, but leave my tank uncovered in the summer.
I asume by cap, he means the light fixture, but I'm not familiar with the terminology. You would place your light fixture on top of the versa top.
What do you want to do with your saltwater tank? Do you know what kind of filtration you'll want? What kind of animals, corals, invertabrates do you want? These things can affect the overflow, top and lighting design.