Though I have a hard time labeling any weapon of mass destruction as "great," nuclear weapons are certainly incredibly powerful weapons and are capable of creating incredible amounts of destruction.
Their basic power comes from the fact that the weapon's detonation involves using the incredible power built up within molecules by the forces holding individual atoms together and unleashing that power suddenly and forcefully. There are two types generally, fission bombs and fusion bombs though at times they are actually combinations of the two.
The most destructive bombs ever built were called "hydrogen bombs" and are fusion powered bombs that basically create (briefly) a miniature version of the sun. You can imagine how this works out for the area in which it is detonate.