Hydrogen bombs are thermonuclearweapons which employs fusion of isotopes ofhydrogen. The result is greatly increased explosivepower when compared to single-stage fissionweapons.The energy released in a Hydrogen bombis several magnitudes higher than anAtom bomb. A fusion bomb is more sophisticated anddifficult to make, since it requires a muchhigher temperature -- in the order ofmillions of degrees centigrade. So afission is carried out first to producemore energy, which is then used toinitiate fusion. It is easier to make Hydrogen bombs insmall size, so it is easier to place them inmissiles. Hiroshima and Nagasaki both wereatomic bombs and till date Hydrogenbombs have never been used in war.