It's discussions like this that really help us all work through foreign concepts and to iron out the kinks.
A pure carved turn would be described as the tip of the ski and tail of the ski slicing through the same piece of water throughout the turn (nice work @Drago). This is the most efficient turn and one that leads to taking maximum speed into and out of turns. The reality though is that we are quite often not on the ideal line and require adjustments to our line to successfully complete the course. I will go out on a limb and bet that Ted Ligeti (snow ski racer) can carve almost perfect turns down a groomed run without gates but once you place the constraints of the race course on that same groomed run, his pure carved turns would leave him running outside of the course. The same thing happens in waterskiing. When we are skiing on or near the ideal line, the carved turn is amazing and is the goal. Lets look at ways to survive when not on the ideal line...
A smeared turn is one in which the tail of the ski is sliding or rotating outward faster than the rotation of the tip of the ski. To me, optimal placement of Smear comes as the skier approaches the buoy line and still has two hands on the handle. In my world, this is the only place the term Smear applies to my skiing. My ultimate goal is to have the ski set for cross course projection as close to the apex of the turn as possible so that I can maximize my acceleration potential. If I can connect to the boat as close to apex as possible (outside hand back on the handle and tight rope to the boat), I can elongate my acceleration zone by accelerating from a wider point on the boat (Think connecting to the a swing from higher up near 90 degrees). Acceleration ends at the centerline behind the boat, so the only way to gain potential is from the start of the zone.
A skid turn is one in which little to no rotation of the ski has happened in the preturn (ski tip is still pointing outward) so that at the apex of the turn, the skier must forcefully drive the tail of the ski through the turn to create an acceptable attack angle to the next buoy. This action drives the tail deep into the water, relieving the tip (removing ski surface area in contact with the water), so that the tail of the ski will slide through to the appropriate attack angle. The problem with this technique is that the skier is taking an extreme amount of speed into the apex of the turn and then using skidding to slow down and rotate the ski at the most vulnerable moment in the course. Where do the majority of slalom crashes occur? At the finish of the turn before the skier is connected to the boat. While we all utilize skid as a survival technique, this is not ideal.
While writing this, two main differences of Smear and Skid stood out to me. The two main differences are 1) where they happen and 2) that Smear is done with two hands on the handle while Skid is done with only one hand. So one move (Smear) is accomplished in a safe zone of the preturn where falls rarely occur and is accomplished while both hands are connected to the handle and thus the boat. The other move (Skid) is attempted while the skier is fully extended and exposed to falling in the most common crash zone while only connected to the boat by one hand.
These are just some morning thoughts while having coffee.