Talk to Me Like I'm Stupid: The Bomb, Baby (Winner)

We have a rather simple one this week, if somewhat embarrassing. (All of these are embarrassing.) How does a nuclear bomb work? Much like derivatives, I've read some on this and I have a rather basic understanding. I understand that this about splitting atoms.  But even after reading a really great biography of Oppenheimer, I don't have a real grasp of how a nuclear bomb actually works. More importantly, and more relevant to today, I have little understanding of why it's difficult to make a bomb. I don't really get the relevance of yellow-cake, or why uranium is important, or why you can tell by looking at a few rods that a country is pursuing a nuclear bomb.

As last week, I'm looking for answers that can transcend jargon (to the extent that that's possible) and translate the complicated into something accessible for the lay reader. Extra credit for anyone willing to delve into policy. For instance, whenever I hear people talk about nuclear containment and preventing, say Iran, from getting the bomb, I think, "If I were an Iranian, whatever my thoughts on Ahmadinejad, I would want the bomb." Why would I trust America to be the ultimate steward? What gives them a special line to God? But then, I'm not Iranian, so maybe I'm missing something.

At any rate, have at it. We did well last week.

UPDATE: Guys, I'm going to prune the comments just a bit. No one's said anything offensive. But these threads tend to go long. I want people to have to press "Load More Comments" as little as possible. Sorry for the inconvenience. No disrespect is intended.