Rear-wheel Drive
Rear-wheel drive (RWD) is when the driving force is applied only to the wheels at the rear of the car. RWD vehicles usually include sports cars, vans, trucks and older cars. These vehicles are at a disadvantage in winter conditions because the bulk of the weight in a vehicle is the engine, meaning the rear of the vehicle is less secure on the road and more prone to slide. It is best to keep some kind of weight in the back of your vehicle, such as sand weights made specifically for this purpose or other heavy items like cinder blocks. Having a full tank of petrol will make a difference as well, as the tank is usually situated between the rear wheels. Although it doesn't provide a great amount of excess weight it might also be prudent to keep a bag of cat litter or road salt in the back, if you become stuck you can use the contents to provide some traction.
Front-wheel Drive
Front-wheel drive (FWD) is infinitely better in bad conditions because the driving force is applied to the front wheels where the weight is, improving your manoeuvrability and stopping ability. This doesn't mean you will never lose control of a front-wheel drive car, it means your chances of losing control are significantly less if you are driving responsibly.
Four-wheel Drive or All-wheel Drive
The first thing to know about these vehicles is that they do not make you invincible. The false sense of security these vehicles provide lead to a disproportionate number of them being found in ditches when the roads start to ice over. But they do provide better manoeuvrability, because when all four wheels receive power, they provide better traction and are less likely to slide. And as all wheels are driven, the main driving force (as with FWD vehicles) is in the front where the bulk of the weight is. Also, as four wheel drive (4WD) and all-wheel drive (AWD) vehicles are usually (but not always) trucks and SUVs, they will be of heavier construction all around, and larger vehicles provide more protection in the event of a crash.