Fusion energy is the extraction of energy from bonds between particles in the nuclei of atoms by fusing those nuclei together. To gain the most energy, light elements and isotopes like hydrogen, deuterium, tritium, and helium must be used, though every element with an atomic number lower than iron can produce net energy when fused. Fusion is in contrast to fission, the process whereby energy is generated by breaking apart heavy nuclei like uranium or plutonium. Both are considered to be nuclear energy, but fission is easier and better developed. All present-day nuclear power plants operate based on fission energy, but many scientists are hopeful that a power plant based on fusion energy will be developed before 2050.
There are nuclear bombs based on both fission energy and fusion energy. Conventional A-bombs are based on fission, while H-bombs, or hydrogen bombs, are based on fusion. Fusion more efficiently converts matter into energy, producing more heat and temperature when the process is channeled into a chain reaction. Thus H-bombs have higher yields than A-bombs, in some cases more than 5,000 times higher. H-bombs use a fission "booster" to attain the required temperature for nuclear fusion, which is approximately 20 million degrees Kelvin. In an H-bomb, approximately 1% of the reaction mass is converted directly into energy.
Fusion energy, not fission, is the energy that powers the Sun and produces all its heat and light. In the center of the Sun, approximately 4.26 million tonnes of hydrogen per second is converted into energy, producing 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This sounds like a lot, but it's actually quite mild taking into account the total mass and volume of the Sun. The rate of energy production in the Sun's core is only about 0.3 W/m3 (watts per cubic meter), more than a million times weaker than the energy production that takes place in a light bulb filament. Only because the core is so huge, with a diameter equivalent to about 20 Earths, does it generate so much total energy.
For several decades, scientists have been working towards harnessing fusion energy for the needs of man, but this is difficult because of the high temperatures and pressures involved. Using fusion energy, a unit of fuel the size of a small ball bearing can produce as much energy as a barrel of gasoline. Unfortunately, all attempts at fusion power generation as of 2008 have consumed more energy than they have produced. There are two basic approaches -- use a magnetic field to compress a plasma to the critical temperature (magnetic confinement fusion), or fire lasers at a target so intense that they heat it past the critical threshold for fusion (inertial confinement fusion). Both these approaches have received significant funding, with the National Ignition Facility (NIF) trying for inertial confinement fusion and coming online in 2010, and the International Thermonuclear Experimental Reactor (ITER) trying for magnetic confinement fusion and coming online in 2018.