The most common components of a roof rack system are: towers, fitting pieces (for attaching towers to a specific vehicle), crossbars, and gear mounts.

Automobile roof racks are split into different types, depending on the vehicle roof:

Rain Gutter – older roof racks were usually mounted directly to the gutter surrounding the roof line.
Bare Roof – many modern vehicles, which do not have gutters, can have a roof rack installed by attaching hooks to the top of the door frames.
Fixed Point – some automobiles have fittings for proprietary racks which mate with reinforced lugs in the roof, or have pre-threaded screwholes.
Side Rails – vehicles with factory-installed rails, which may be flush against the roof or raised off of the roof, running front-to-back on the roof
Factory Bars – other vehicles have a factory-installed permanent roof rack.