A hydrogen bomb is a nuclear weapon that relies on the principle of fusion to generate its deadly blast, which is far more powerful than what could be generated from a bomb that relies on the fission of uranium or plutonium.
Also known as a thermonuclear bomb, it works on the principle of the fusion of two nuclei, and generates temperatures similar to those found at the sun's core.
When such a bomb is detonated, chemical, nuclear and thermonuclear explosions succeed each other within milliseconds. The nuclear explosion triggers a huge rise in temperature that in turn provokes nuclear fusion.
A hydrogen bomb has a yield measured in megatonnes, whereas fission weapons typically have a yield measured in kilotonnes. One megatonne is equivalent to 1,000 kilotonnes.
To put that in perspective, the nuclear bomb dropped by the United States on Hiroshima during World War II had a yield of about 15 kilotonnes.
A hydrogen bomb is "cleaner" than an atomic bomb as it results in less radioactive fallout. It can be fitted in the warheads of ballistic missiles. Only a few countries are known to have hydrogen bombs: the US, Russia, Britain, France, China and India. Israel is strongly suspected of possessing hydrogen bombs, but it has yet to confirm or deny this.