Fission = The splitting of the atom, the most common form of nuclear reactions (every atom is trying to decay to Iron, fission is taking a large atom and make it a smaller atom)
Fusion = Combine atoms together, not as common as fission because the atom is getting large.
For Nuclear Reactors
There are currently no nuclear power plants that use Fusion.... fusion reactors are a sort of holy grail for nuclear power plants, over a billion of dollars have been spent trying to create a fusion power plant, the largest of these projects is the tokamak). Why we (as scientist) obsessed about fusion power.
- Safety, the fusion reactor is supposed to not produce radioactive waste
- Cheaper, fuel for the fusion reactors is plentiful, with salt water being the preferred source.