Okay, so if you don't know what a fusion reactor is or how it works I think you are probably in the wrong department. However, since I am here and you are here, we might as well discuss it. This is your basic fusion reactor.
Starfleet uses a simple Deuterium plasma induced reactor. This works completely different than a laser induced reactor, such as the Cardassians use. The deuterium is forced through the inner chamber of the reactor. The force and speed at which the deuterium is forced through the chamber creates a super heated plasma. The plasma forces the deuterium to combine creating heavy helium. This fusion releases energy.
This is the inside of a fusion reactor. The plasma is forced into this chamber and travels around it with the aid of magnetic fields.
The process gives off radiation that slowly builds up over time. The use of dampening and containment fields are required to protect the station and its personnel. Proper maintence is also required for efficient energy output. If we maintain these beautiful machines they can last for quite some time, and will provide the station with plenty of power.