This week has not been the best week for driving. It's been very snowy, and so cold that road salt has been working very poorly, making the roads a slippery mess. Loss of traction can cause serious accidents, so it is best to drive slowly. In the end, it all comes down to friction. Whether you're fishtailing, or stuck in a rut, or have gone into a full on skid, too little friction can cause serious problems.
Most commonly people will end up fishtailing more in snowy weather, usually while going around turns. This is characterized by a loss of control in the rear wheels, where they begin to slide as the car continues to turn, even when you try to straighten out. The scary part about sliding is, however, that it is harder to stop than it is to prevent. Once you've started to slide, frictional forces tend to oppose you, and your best bet is to point your front wheels in the direction of the skid and wait until you've regained traction. A similar problem occurs when your drive wheels are spinning out, skidding without a solid connection with the ground. A result of accelerating too fast, it is best remedied by easing up and taking it a bit slower next time. But worst of all is when your front wheels begin to lose control, preventing you from steering and often sending you into a spin. Like with all other winter driving problems, you just have to easy up and "go with the flow", until you can regain control.
Proper winter driving requires a gentle touch, and grace under pressure when the inevitable happens. On bad days it is almost unavoidable to have some slipping here and there, but as long as you don't try to overcompensate the issues will stay minor and not wind up getting much, much worse. So drive safe, and try not to panic.