This applies to driving in rain and ice just as much as it does snow.
Traction is the friction that exists between your car's tires and the surface that your car rests on. Whenever you take an action with your car (accelerating, braking, or turning), you are using some of the traction that exists to accomplish this. If you take extreme action, you may lose traction. For example, floor the gas when starting and you may peel out and not go anywhere. Slam on the non-ABS brakes and your wheels will lock. Turn while you are going too fast and you will slide. Generally, these are undesirable actions.
As your car's speed increases, you need more traction to turn. This is because a faster car has more inertia in the previous direction that must be overcome. Therefore, it is easier to straighten out a slower moving car than a faster moving car.
One way to look at driving is as a series of control inputs. The driver has a limited amount of basic control inputs to the car: accelerating, braking, and turning. Any one of these control inputs uses traction. If you find yourself in a skid because of bad weather, it is important to use the smallest amount on control inputs to get yourself back on track.
Do not brake and turn at the same time as one needs all the traction available to be focused on stopping the car from going into bad places. Turn into the skid and do not jerk the wheel, but rather turn it slowly and deliberately.
All the textbook knowledge in the world won't prepare you for a skid as much as experiencing one with no consequences. While there aren't many (if any) places in the city, a sufficiently iced-over lake in a rural area will do for practice.
By the way, this is all my opinion and it is to be taken with a grain of salt.