How do nuclear weapons work?

How do nuclear weapons work?

I suggest reading Richard Rhodes books: The Making of the Atomic Bomb and Dark Sun.
Wikipedia also contains good explanations.
There are a number of different types that operate by different principles:
1. Fission: splitting large unstable atoms into smaller ones with neutron chain reaction.
2.
1. Gun design, always uses Uranium, fires a bullet and target together with cordite down a cannon barrel to assemble a supercritical mass followed by a pulse of neutrons to start the chain reaction.
2. Implosion design, can use Plutonium, Uranium, or a composite of both, shaped charge high explosives crush this core to assemble a supercritical mass followed by a pulse of neutrons to start the chain reaction.
3. Fusion: joining small atoms into larger ones at high temperature and pressure. Note that pure fusion bombs (with no fission component at all have never been built).
4. Mixed Fission-Fusion: most modern bombs use some mixture of the reactions to optimize the weapon for the mission, the delivery vehicle, and production costs.
Fusion boosted low yield tactical fission weapon. These use a small amount of tritium gas inside the fission core to boost efficiency of a tiny low yield core with extra neutrons from the tritium fusion.
Multi-stage high yield fusion weapons. These even raise yield by fissioning the normally nonfissionable Uranium-238 isotope with high energy fusion neutrons.
etc.