The hydrogen bomb is the single most destructive weapon ever devised by man, and is the only successful effort by mankind to harness the same basic process that is created deep inside the sun to generate energy. The effects of a hydrogen bomb are essentially the same as those created by any nuclear weapon--heat, blast, and radiation--but on a much larger scale.
Hydrogen bombs are a combined fission-fusion nuclear device. There are a wide variety of designs (some of which have nothing to do with hydrogen), and these vary in their particulars but share some general features. Thermonuclear weapons of this type use a normal atomic bomb, which uses nuclear fission (or atom-splitting) to provide the energy to provide the heat and pressure to create a nuclear fusion reaction.
Power and Size
Hydrogen bombs were part of a general effort to develop ever more powerful nuclear weapons, and indeed the most powerful nuclear weapons ever built are all of the fusion type. The infamous Tsar Bomba, for example, was a hydrogen bomb test-detonated by the Soviets, and continues to stand as the single most powerful nuclear weapon ever built or detonated. It had a staggering yield of 50 megatons. However, the design type also allowed for nuclear weapons that could pack a considerable wallop in a small package. The U.S. W-47 warhead of the 1960s, which was deployed on submarine-based nuclear missiles, packed up to 1.2 megatons but only weighed about 725 lbs and was small enough to fit on the Polaris missile. By comparison, the primitive fission bomb dropped on Hiroshima had a 15 kiloton yield.
A little more than a third of the energy from a hydrogen bomb is released in the form of heat, light, and some of the softer forms of hard radiation, such as ultraviolet and X-rays. The first and most immediate result is either temporary or permanent flash blindness on the part of anyone who was looking at or in the general direction of the blast without proper eye protection. The energy release also creates enormous temperatures, especially in the case of a powerful hydrogen bomb. A hydrogen bomb can create temperatures in a range that is 6,300 times hotter than the surface of the sun. This routinely vaporizes much of whatever matter is immediately around "ground zero" (the center of the explosion), fuses dirt and sand in the ground into glass, and produces a mammoth fireball.
When a normal nuclear weapon other than a neutron bomb is detonated, about half of its energy is expressed in the form of the concussive blast. All explosions cause this effect because the heat released creates an overpressure, or a wave of greatly increased atmospheric pressure generated by the explosion and that radiates out from it. The difference between a hydrogen bomb and another type of explosion, even a nuclear explosion, is the substantially greater blast energy. An interesting side note to nuclear blasts, however, is that they are dependent upon an atmosphere to propagate them. Contrary to what a lot of bad science fiction might have us believe, the vacuum of space would eliminate this concussive blast, leaving on the remaining two nuclear effects.
About 15 percent of a hydrogen bomb's energy takes the form of radiation. About 5 percent takes the form of ionizing radiation, or highly charged particles and gamma rays that are emitted as part of the fission and fusion chain reactions in the hydrogen bomb. The remainder takes the form of nuclear fallout. Fallout is the spread of radioactive materials (waste byproducts and unspent fuel from the bomb) through the atmosphere as a result of the explosion. These substances continue to emit dangerous radioactivity as a result of their own radioactive decay.