In the fictional Star Trek universe, the holodeck is a form of virtual reality. It is an enclosed room with force-fields (which are similar to shields and deflector fields), onto which light is projected, allowing the simulation of three-dimensional surfaces of objects (including people). The effect is a simulation of entire environments, with which the user can interact. Two of the main purposes of the holodeck are for recreation, and training.
The holodeck includes safeguards designed to protect the safety of the user or users.
Despite the frequency at which holodecks have malfunctioned (as a plot device, holodeck malfunction is right up there with transporter malfunction), Starfleet has apparently not taken the initiative to disable and remove them.
