All cars have a frame, of one form or another. The frame bears the weight of the car and provides the foundation of its structural elements. The traditional assembly line invented for the Ford Model T bolted everything onto the frame. The Model T used a ladder frame. Ladder frames consist of two thick longitudinal frame members, usually square tube or I beam shaped, with a series of connecting elements across them so they resemble a ladder from above. Ladder frames dominated car construction for a half century and are still used for trucks because they are inexpensive, and have good longitudinal strength, bear weight well, and don't take up much interior space. They are fairly simple to repair.
But ladder frames have a problem. They like to twist. An engineer would tell you they have poor torsional rigidity. And they bow a bit under load. Not a big deal if you're hauling logs. Engineers calculate the expected load and design so the frame sags into the desired position (check out an empty flatbed trailer for an example of this). But for racing, or any other form of high performance motoring torsional rigidity is a big deal. Because under heavy load, the car itself can act as a spring.
This is a very bad thing. The purpose of any automotive suspension system is simple: Keep The Wheels On The Ground. After all, the car goes stops and turns entirely through the four contact patches where the rubber meets the road. If the world were perfectly flat and smooth, that job would be simple. But even the smoothest race track has bumps, and dips, and changes in camber. For this reason spring rates and sizes, suspension travel, shock absorber ratings, bushing types, and even the overall design of the suspension are the process of careful, painstaking development. all to keep those wheels perpendicular to, and in contact with, the road.
But if the frame or body flexes, all that development means nothing, because the relationship between rubber and road is no longer under the control of the engineer. Control arms are designed to operate at a precise angle, if the body flexes they are no longer at that angle, and therefore the tire is in the wrong place. Traction is lost. Body flex also tends to be abrupt and violent, which makes it very difficult to account for even when the flex point is known. Which makes the car almost impossible to set up.
One solution chosen by designers is monocoque, or stressed skin frames. This was pioneered by Lotus engineer Colin Chapman for his Formula 1 race cars, but the first practical example was the Blood Brothers Cornelian, a car produced in the early 1920's. Unibody construction is what monocoque is referred to in passenger cars, and was pioneered by Chrysler. Essentially the body is the frame. If you look at the body shell of a modern street car, you note that the roof, door sills, floor, transmission tunnel, etc are all one piece. The stresses acting on the car are transferred through the floor and roof through the A, B and C pillars. Such design produces a frame that is internally roomy and can be very strong for street use, particularly when the doors are shut. (This structure also explains why convertibles need heavy reinforcement if they are not to shake all the time. The roof contributes immensely to a sedan's strength). But they still flex too much for racing.
In a race car, the car will be a tub, upon which other components will be hung. Starting with the 1966-7 Lotus 43 Indianapolis race car, the fastest race cars ( CART, Formula One, Can Am, etc] were made with semi-monocoque construction, where the skin itself is stressed. Often the skin also served as fuel tank, or fuel cell enclosure. The Lotus 49 Formula 1 car added the innovation of using its Ford Cosworth DFV engine as a stressed member. This is the lightest, and if properly designed, the stiffest possible construction. They are enormously strong. For example, the Champ Car Lola or Reynard tub is made of carbon fiber and literally bulletproof. Yet it collapses nicely to absorb energy during an impact.
However, such tubs are expensive and difficult to successfully design. Plus they are very difficult to repair after an impact. Often the tub must be thrown away. Only very well financed race teams can afford such expense. Lesser, but still serious, racers go with a tube frame.
Tube frames are constructed of either square or round tubing, and of chrome moly or mild steel. The roll cage (or bar for open top cars) is integrated into the frame as a structural element. A tube frame will have four longitudinal tubes running the length of the car. Two lower tubes will be in the same position as in a conventional ladder frame, but two will be much higher and linked through a series of tubes. It is this four tube setup which gives a tube frame the torsional rigidity the two member ladder-frame lacks. Tubes will be enjoy triangular reinforcements for strength, as triangular shapes are very strong. The engine, driver and transmission are inside the tubes, body, suspension and other stuff hung from the tube. Often interior walls (usually aluminum) are used to divide the mechanical sections of the car (where the firey stuff is) from the driver's compartment.
Tube frames are inexpensive and simple to construct, easy to repair and don't require an engineering staff to design. All you really need is a good welder and a computer with finite element analysis software. Not that I'd recommend skipping the engineer. A good tube frame is very stiff. Even if tubes are added, a car retaining its unitbody will lose its setup often, requiring more suspension adjustments. They offer good driver protection and tolerate damage well. Field modifications are easily accomplished. For this reason they are popular with both professional and amateur racers, including NASCAR's Winston Cup series. Formula Ford, Continental and Formula V cars are all based on the tube frame, as are most Grand Am cars.
Tube frames are rarely used in street cars because the top tubes and triangulation make for a very restrictive interior. But for racing, they are excellent.