Safety Car (also S/C or just SC) refers to a car, which may enter the circuit during a session, thus bringing safety car conditions into effect.
The principle effect of the safety car is to reduce the speed of the cars, and to bunch them up into a single 'train' which allows marshals to perform otherwise dangerous work, without stopping the session entirely. The safety car may be used under other circumstances too, such as when conditions are dangerous (e.g. heavy rain).
The safety car crew consists of a 'competent' driver, and 'safety car observer,' who is, typically, an experienced post chief.
In addition to bringing a session under control, the safety car may be used as a pace car for rolling starts, and as an escort vehicle to bring racing cars from the assembly area to the starting grid.