First, let us bow our heads and give brief thanks:
"We thank thee, creator of roads and corners. Bringer of snow and handbrakes. You alone make driving fun."
And now, lets talk about driving in the snow! Effectively, snow lowers the coefficient of static friction between the tires and the road surface. This means that for the same weight of car and contact patch with the road, The road can provide less "push" against the tires, so they're more likely to spin instead of roll.
This effect is increased in corners, due to the added force applied to the tires. Rather than simply being asked to provide force in the plane of the wheel, they're now also being asked to provide force in a plane perpendicular to the wheel. Vector addition tells us that if we add two perpendicular vectors, the resultant is the square root of the individual vectors squares (Pythagorean theorem).
This spinning instead of rolling is also 'helped along' by using the car's brakes. Fortunately for safety, but unfortunately for someone who actually enjoys skidding, modern cars are equipped with antilock brakes (ABS). This means that if a wheel starts to slide under braking rather than continue to roll, the car automatically pulses the breaks, working to find the point where maximum breaking force is applied to a wheel without causing it to skid. As evidence that there really is a divine being, most cars are equipped with a hand-operated emergency brake. This brake has a direct connection to the wheels, and is therefore unaffected by the antilock braking system.
In the end, maximum slide is achieved by going around a corner and then pulling the handbrake. This ensures that the tires are under maximum load, and then knocks them loose by locking them up.
As always, be safe, don't wreck any lawns or mailboxes, past results do not guarantee future success, don't drink and derive, and don't share needles.