Atomic vs. Hydrogen Bomb
Atomic: Involves fission reaction, in which a neutron collides with an atom's nucleus, splitting it into two nuclei and releasing nuclear energy. Also called an A-bomb. Nuclear reactors use fission to produce electricity. The U.S. dropped atomic bombs on Hiroshima and Nagasaki, Japan, in 1945 to bring an end to World War Two.
Hydrogen: Involves fusion reaction, in which nuclei collide to form a new nucleus. Also called a thermonuclear bomb or H-bomb. The sun and stars are powered by the fusion process. Fusion reactions allow for massive explosive yields – thousands of times more powerful than an atomic bomb.
Comparison: The atomic bomb dropped on Hiroshima produced an explosion equivalent to 13,000 tons of TNT. "Ivy Mike," the first thermonuclear (hydrogen) test, was carried out on the Enewetak atoll in the Marshall Islands in 1952, and produced an explosion equivalent to 10.4 million tons of TNT.
Source: Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization