There is no salient difference when one goes of on your home town. But the route that they were obtained is important in this case.
To develop a plutonium bomb you need a reactor, a specific kind of a reactor, that NK has to irradiate uranium and turn it into plutonium. As they are different chemical elements, chemestry seperates them. So once you have irradiated uranium getting the plutonium is an easy step.
To get weapons grade uranium for a uranium bomb things are a bit different. You need to seperate isotopes of the same element. They are chemicaly identical so no chemestry can seperate them. Theyre is a sublte weight difference though. The fraction of 238/235. Various methods are used to seperate the two differing weights of isotopes.
http://en.wikipedia.org/wiki/Gas_centrifuge gas centerfuges are the easiest to attain innitialy, but they are energy hungry and take hundreds to make a feasable weapons program as they tend to only be able to make milligrams of U235 per year per unit.
North Korea had obtained about 12 (not sure of the exact number) a number perhaps 30-40 times too small for a weapons program. But the Americans accused them of developing Uranium bombs.
The difference in how they are detonated is also salient. The uranium bomb requires a relatively unsophistaceted 'gun' type assembly method. Basicaly a slug of uranium is shot out of a gun into a larger mass pushing it above critical mass. Almost anyone who can build an artillary gun can produce this kind of weapon,
with weapons grade uranium.
Plutonium bombs however will not work like this. They will predetonate and fissle. They require explosives to surround them and compress them to push them above a critical density threshold. This is a rather more difficult technical challange and requires some no mean engineering and physics to get right first time. In part because of the nuclear physics and getting the wrong kind of explosives can also fissle the weapon.
Why this is important is that Korea had enough plutonium from its reactor to make a plutonium bomb or two. Far far to little to test its design with. So it had to be very conservative with its design to hope it worked. However once the Agreed Framework collapsed it gained access to alot more plutonium it already had and is now able to refine far more complex weapons it can mount on its small rockets instead of lumping great monsters that would struggle to fit aboard a C-130 Herculeas.
