A roll cage is a specially constructed frame built in (or sometimes around) the cab of a vehicle to protect its occupants from being injured in an accident, particularly in the event of a roll-over. Roll cages are used in nearly all purpose-built racecars, and in most cars modified for racing. There are many different roll cage designs depending on the application, hence different racing organisations have differing specifications and regulations. They also help to stiffen the chassis, which is desirable in racing applications.
A roll bar is a single bar behind the driver that provides moderate roll-over protection. Due to the lack of a protective top, some modern convertibles utilize a strong windscreen frame acting as a roll bar. Also, a roll hoop may be placed behind both headrests (usually one on older cars), which is essentially a roll bar spanning the width of a passenger's shoulders..
A newer form of rollover protection is deployable roll hoops that are normally hidden within the body of a car. When sensors detect an imminent rollover, the roll hoops quickly extend and lock in place. Cars that have a deployable rollover protection system include the Peugeot 307 CC, Mercedes-Benz SL 500, and Jaguar XK.
(fonte Wikipédia)
