just looking for some BASIC knowledge about why its important to have a good radiator cap...

To keep the coolant from splashing out.

No, just kidding. Raising the pressure inside the cooling system raises the boiling point of the liquid (water/anitfreeze). Helps prevent boiling over but also helps prevent localized hot spots from starting to "sizzle" inside the engine and degrade the cooling system's ability to carry heat out of the engine.

ok, well i just bought a new aluminum universal fit radiator from Pepboys, and it has the little overflow nipple on the side of the neck that goes to the overflow tank. Right under the water neck, about 2 inches down, pointing towards the engine, there's another little outlet that doesnt have threads or anything... what's this for? and its open, not pluged...i should plug it right?!?! with what... sorry for the dumb questions!!! Water shoots out from there when the engine gets too hot!!!

It should have a plug on it or in it if its threaded.
If its a nipple like a heater hose, a plug and hose clamp will do it.
It should be on the radiator when made.

Radiator caps are to keep the bugs from getting in there, everyone knows that.