The complete evolution of the seat belt is the five point seat belt, or harness. They feature a strap over each shoulder, a strap around each side of the body, and a crotch strap. All of the straps come together over your belly and latch there; One either turns a knob or slaps a button to release it. There are also four-point harness-type seat belts, and six-point, but the five point types are the most common.
Five point belts are required for most classes of racing. The shoulder belts usually strap to a roll cage or roll bar behind the seats, the side belts to mounting bolts beside the seat, and the crotch strap to a bar running between the front bolts holding down the seat itself. Five point belts are generally sufficient to keep you in your seat even if you roll the vehicle. Five and six point belts both serve to keep you from submarining, or sliding down towards the toe panel in a head-on collision, but six point belts have the added advantage (especially to male drivers) that they do not tend to crush the crotch region if such a thing should happen, as the fifth belt in five-points runs straight down from the clasp, bisecting the crotch region, potentially more literally than one might appreciate.
The US Code, or United States Code of Federal Regulations, mandates certain requirements for seat belts in Title 49, Volume 5, Part 571. Every racing organization, including the FIA, NHRA, SCCA, and so on, have more stringent requirements than the US government. For example, modern cars are required to have both lap and shoulder belt for each front seat passenger, and a lap belt for the rear passenger. Seat belt straps are required to be made out of webbing, or a woven material, to enhance their flexibility and lifetime. The webbing must be at least 46mm wide, support 26,689 N for a lap belt alone, or 22,241 N for the lap belt, and 17,793 N for the shoulder belt. They are tested for their resistance to abrasion, light, and corrosion. The buckles have to meet certain characteristics for how hard it may be to press the button, how hard it can be to slide the buckle closed, et cetera.
The FIA, by contrast, requires a four, five, or six-strap belt. The FIA specifications for a five or six point belt include crotch and pelvic straps at least 44mm wide where they cross the thigh, and at least 25mm elsewhere; Other straps have to be at least 70mm wide, considerably wider than the US Code requirements. Also, the buckle is required to have between 20 and 40cm^2 of contact with the wearer, to spread out any impact there. Other requirements are substantially higher.
The downside to five point belts is that they take more time to put on, and they will do you very little good if you do not connect all of the belts, as they are designed to work with all of them securely fastened. Proper adjustment is even more important with five point belts than with normal lap or shoulder and lap belts. They also reduce your freedom of motion. However, this reduction also makes you considerably safer, and you shouldn't be doing anything while you're driving that you can't do while strapped in with five points, either. In the US, as long as they are listed with the [U.S. Department of Transportation (USDOT), there is no case for them not being street legal as long as you follow the statutes of 49CFR571.209, Seat belt assembly anchorages.
References:
49CFR571.209 Code of Federal Regulations Title 49, Volume 5. October 1, 2001
From the U.S. Government Printing Office via GPO Access