Understeer is basically a skid where you are turning the steering wheel, but the car keeps going straight, or is not turning as quickly as you would like. Understeer is the characteristic that engineers build into most cars, particularly front wheel drive cars. The reason for this is that your fear response is the correct one.
Video below, which comes via oppositelock (via), perfectly illustrates the dangers of being surprised by understeer in 1970 Saab 96. The Video takes us inside a Saab 96, a 1970s machine that relies on its front wheels for traction and has never heard of ABS. The driver is faced with one of the worst possible scenarios one can encounter on the road. To be more precise, the man’s Saab has to go through a downhill turn with a decreasing radius, one that also appears to have a bit of negative camber. As the driver starts turning vigorously in order to go through the twist, the vehicle starts going wide. A rather basic driving instinct tells the man to step on the brakes, which only makes the problem worse. Luckily, the guy realizes his mistake before the situation becomes an offroading adventure and takes his foot off the brake. The front tires start handing the steering job properly shortly after, with the Saab being able to return onto its lane.