Initially, nuclear reactors were designed to specifically produce plutonion for bombs. At present, however, the primary use is that of electricity generation. A conventional reactor assembly consists of the fuel, coolant, moderator, and neutron absorption materials. There are a number of reactor types: PWR (Pressurized Water Reactors), BWR (Boiling Water Reactors), PHWR (Pressurized Heavy Water Reactors), LMFBR (Liquid Metal Fast Breeder Reactor), and others.
FAS Resources on Nuclear Reactors and Associated Technologies:
Bodanksy, David, Nuclear Energy: Principles, Practices, and Prospects. New York: Springer, 2004.