There are four different ways to move downhill : sliding, slipping, skidding and carving.
Sliding - skis moving downhill in the direction they are pointed. This can be in a straight run down the hill or a traverse across the hill.
Slipping - skis moving sideways down the hill at an angle relative to the long axes of the skis. The direction of travel is perpendicular to the skis.
Skidding - is a combination of sliding and slipping as the skis move through the turn. Tails of the skis making a wider path than the tips. Most turns involve some amount of skidding.
Carving - when tips and tails travel through the same arc created on the snow.