Here is the wikipedia entry on the Trinity. Now you all understand it perfectly, right?
My favorite Trinity story came from Eldridge Cleaver (not related to Wally and Beaver). He went to a class on religion in which the nuns asked him to give his understanding of the Trinity. The idea was supposed to be to show that no one could truly understand the Trinity. Only, he thought he did understand it. It was like Three-In-One Oil.