There are two major types of nuclear weapon/bomb
An atomic bomb
A hydrogen bomb

An atomic bomb works by splitting large atomic nuclei (fission) such as uranium or more usually plutonium, this releases quite a lot of energy.

there is however a limited size you can make this as the bomb tends to blow itself apart before it has all ignited.

Hydrogen bombs use an atomic bomb to ignite a nuclear fusion bomb - this is fusing hydrogen istopes together to form helium. This releases a lot more energy, especially as the neutrons released by the fusion part of the bomb makes the fission part more efficient.