When referring to cars, a spinout is one of the things that can happen when you enter a curve at a too high speed.
A spinout occurs when the rear tires lose traction during a turn, which results in the car pointing further in the direction of the turn than the driver would like. If the driver is able to correct this situation, what you get is a drift, or a skid. If the driver is unable to correct the situation, the car eventually ends up pointing in a direction that's totally inappropriate, and the car as a whole ends up stopping in the road pointed in the wrong direction, or sliding towards the outside of the turn - hopefully onto a wide expanse of grass, and not into a concrete wall.
Because spinouts occur when the rear tire loses traction before the front tires, spinouts typically occur in rear wheel drive cars that are trying to accelerate more than they should be during a turn. Also, because the act of braking transfers weight to the front of the car (reducing rear traction), spinouts can also happen with too much braking during the turn.
Spinouts can also occur through no fault of your own, for mechanical reasons, or if another vehicle hits your car during a turn.
You can avoid spinouts by not going too fast in a curve, getting really good tires, getting a really high-performance vehicle, or not turning. Trying to avoiding spinouts by driving a front wheel drive vehicle may result in understeer. You can still spinout in a front wheel drive car anyways.
Also referred to as oversteer (which also may encompass drift).
Apparently, you're supposed to hit the brakes during a spinout, hoping to slow your car down before it hits something...