While the antilock brake system (ABS) helps keep traction while braking, and traction control (TC) helps keep traction while accelerating, it's yaw control that helps you keep traction while turning (or trying not to!) All three systems watch the output of the wheel sensors and take actions based on them; ABS tries to make sure that all wheels are turning at about the same rate, and TC tries to make sure that your use of the throttle doesn't cause them to spin, but yaw control's job is to make sure that the car goes where you are trying to make it go, and that makes it the most complicated out of the three.
Of course, I speak of them as separate systems, but in reality they are all part of the same thing. Even if the systems are housed in different computers, they all tend to get their wheel speed information from the ABS controller and they all get information on vehicle speed, engine load, or whatever else they might want to know from the powertrain control module (or "PCM".) The biggest difference between yaw control and traction control is that the yaw control contains an accelerometer. Today, this is usually a MEMS device consisting essentially of a weight on the end of a spring whose deflection can be measured. Such a device can measure static or dynamic acceleration — that is, either gravity or motion respectively. Of course, we're interested in detecting motion, and if you imagine impaling the car on a spike from above, the computer needs to know when the car is rotating about that particular axis.
When rotation is detected, the car might decrease the throttle, engage the brakes on a particular wheel, or even do both. The vehicle typically also monitors the position of the steering wheel, so that it knows when you are trying to turn the car. It would seem like a difficult task, and it is, but the system can be amazingly helpful. Reviews of the fully-loaded Corvette Z06, for example, praised the car as being almost impossible to "lose" when the traction and yaw control were in use. Probably the cheapest vehicle with yaw control is the Subaru Impreza WRX, which also has a number of other interesting features not typically found on street vehicles.
It is worth noting that like ABS, yaw control probably does not reduce accidents. Features such as these DO make vehicles safer, but people tend to push limits, and it tends to cause people to drive even more rapidly and unsafely, trusting that the car will rescue them. These features are not magical, and forgetting that is perilous.