A roll cage is a specially constructed frame built in or around the cab of a vehicle to protect the 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 specifications of the sanctioning body. Roll cages are built to extend the frame in front of the driver, next to the A-pillar, to provide as much high-speed protection as possible in a coupé. This is compared to the protection provided in open wheel racing, where a solid shell encases the majority of the driver's body as well as a rollover hoop that extends over the driver's helmet. A roll cage can also do a great deal 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. A targa top is a body style that integrates an externally-visible roll bar that a semi-convertible top attaches to. Convertibles have particular concerns for roll-over protection. In some modern convertibles a strong windscreen frame acts as a roll bar.
Note about potential confusion: roll bar is also the name of a device for stiffening the suspension of a car and is as such sometimes referred to as a sway bar.
Roll hoops are essentially two roll bars, one behind the driver and one behind the passenger. Sometimes just a single roll hoop behind the driver is used. Sometimes they are integrated into the design of the body.
A newer form of rollover protect 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