The Reaction Control System provides attitude control and small translational ability for the orbiter.  Since space has no air, the aerodynamic control surfaces on conventional aircraft are useless in space, and in turn small rocket engines, or jets, are used to provide this ability.  The RCS also allows the vehicle to make small, precise motions, which are vital to docking.  To use the larger OMS engines during a docking operation would be similar to trying to line yourself up in a parking spot at 80 miles and hour.

    RCS jets pods are located throughout the body of the orbiter, providing the most fuel efficient points for controlling the vehicle.  The RCS jets are fully redundant; if a specific pod malfunctions other pods can provide the same functionality, just not in as full efficient a manner.

    In the event that the OMS system malfunctions, the RCS is capable of using the OMS' fuel supply, allowing it to provide the necessary deceleration for reentry if possible.

    Again, the RCS on the Shuttle is an excellent baseline for the orbiter, but the Shuttle's RCS is designed to move around a beast of a spacecraft and is overkill for my vehicle.  The Shuttle is so large, in fact, that its RCS simply cannot provide the necessary thrust for retrofire if its OMS malfunctions.  Since I am dealing with a smaller vehicle, this redundancy can be regained, as it was used during Mercury, Gemini, and Apollo.