Atomic Bombs VS Hydrogen Bombs
Atomic Bombs use the fission of atoms. Fission/Splitting of atoms changes the original heavy element atom into two or more lighter element atoms and "extra" energy released. This energy multiplied by the number of atoms split releases the tremendous amount of energy!
Hydrogen/Thermonuclear bombs use the fusion of atoms. To fuse two lighter elements into a heavier one requires a lot of thermal energy, to begin with. Hence, hydrogen bombs use the energy from a primary nuclear fission reaction to compress and ignite a secondary nuclear fusion reaction. The bomb gets its name "Hydrogen" because the two lighter elements used mostly are Deuterium and Tritium - both of which are isotopes of hydrogen...