I mentioned earlier the whole idea of mining uranium. At the end of the process there is uranium that is purified enough to be used to produce nuclear weapons, but the main production line is for nuclear energy. In this process—and I have to get just a tad scientific here—uranium itself appears in three different variants. These different variants have a number attached to them. One is called uranium-235, the other one is uranium-238, and there's another one, but I won't get into that. They are different. They're like siblings: they have the same last name, but not the same first name, if you wish, and like siblings, they're different. How are they different, these two forms? They're different in that one of them, which is called uranium-235 and is the important one for creating nuclear energy, is fissile. Therefore it can be used to create energy by a chain reaction, and it is more radioactive. Essentially, those are the two main differences.
The depleted uranium is the residual product that is left after you have separated these two forms, the two variants. The depleted uranium that is left is less radioactive, is not fissile at all, so it cannot be used at all to produce any kind of energy or weapon.