Also called
safety belts. Seat belts are found in
cars and other
vehicles that travel on
roads and have a passenger compartment with
seats. So, you will find them in
large trucks, but not
motorcycles. Seat belts are also part of commercial
airplane seats and the seats of some
boats.
Today, most states in the United States require the driver and any front seat passenger of road vehicles to be wearing their seat belts while the vehicle is in motion. The fine for a violation can be surprisingly high! This hasn't always been the case. I remember riding in the car with my mom in the 60s. She developed an impressive "right arm out" reflex -- anytime she had to brake drastically, she'd fling her right arm out to keep me, or my little brother, or the groceries, from pitching forward.
Older cars had only lap belts -- just one belt that goes from left to right and holds your butt in place. These were better than nothing, but people wearing them still got injured, because their face tended to slam into the steering wheel or dashboard. At some point during the 70s (? anyone remember for sure when?) cars began to be equipped with shoulder belts too, in addition to the lap belt. This additional belt goes over the outside shoulder (left for the driver, right for the passenger) and down to fasten along with the lap belt next to the inside hip. These belts are fairly effective at keeping someone's face from slamming into part of the car in case of a sudden stop, but a new type of injury began to be seen: a sort of brain trauma caused when the head is violently slung to the side. Today (2000), most new cars in the US have airbags, plus lap belts, plus shoulder belts. This can protect you against a pretty serious crash, but airbags are basically explosive and have been shown to seriously injure short people and infants.
It's still a Very Bad Idea to get into a car crash.