What is the difference between a hydrogen bomb and an atomic bomb?

This photo shows the explosion of the Soviet Union’s Tsar Bomb, a hydrogen bomb that was detonated in 1961. The process begins with a nuclear explosion that compresses a sphere of plutonium-239. Inside this sphere is a chamber of hydrogen gas. The high temperatures and pressures generated by the exploding plutonium cause the hydrogen atoms to fuse. This fusion process then releases neutrons, which feed back into the plutonium-239, splitting more atoms and boosting the fission chain reaction.