The Defense Force is an inmate-created and run organization dedicated to the protection of inmates from the wardens, the environment, and each other.
Kitty Pryde kitty_has_claws is the current leader though there are several other team leaders and administrators. See the roster for the full break down.
The mission, purpose, and execution of the DF will likely be interpreted by everyone a little differently and various team leaders have some discretion to act as they see fit. However, Kitty's current goals are to find a way to stop the wardens from bringing more people to the prison and, if possible, return those who wish to leave. She also wants to keep as many people alive along the way as possible which means being ready for inter-inmate conflict and "natural" disasters. She is not interested in petty crimes or squabbles. She also believes in a strong focus on research and investigative methods.
The DF was founded and run by Gino Weinberg uncommonly until he left the dome. It was taken over by Suzaku Kururugi black_as_knight until he also left. In the former prison (Marina classic) the warden supported the DF by providing weapons and life signal readings. This is no longer the case.