Oh, here we go again! Sounds like some people want to make this more complicated than it should be. Yes, I maintain that there really are only two broad categories of ways to reduce speed--"Direction" (going uphill) and "Friction" (increasing resistance to forward motion). Yes, technically, I'm using the term "friction" a little bit loosely here, and simply "increasing resistance" is probably more technically accurate, but "friction" rhymes, so deal with it.
And certainly, there are many sub-categories under these two main headings. But to look at Bushwacker's list, for example, "line" refers to "direction"; "add friction" is, of course, "friction"; "wedge" is a specific way to add friction (or resistance); and pumping or "reverse pumping," whether in bumps or the "virtual bumps" of turns, is a sophisticated way of using "direction" (up or down) with precise timing to generate or lose speed.
And certainly too, we can use them both at the same time. Yes, there is always some amount of friction (and air resistance is just another form of it). But in many ways, they are also mutually exclusive, especially when it comes to the intent behind them (read on below for more about this).
Jamt, I am surprised that you would describe going uphill as "just another variant of friction." The primary technical difference between the two categories is that "friction" and other forms of adding resistance dissipate the kinetic energy of speed into the environment, making it lost to you, the skier. Going uphill, on the other hand, merely converts your kinetic energy to potential energy, so you have not wasted it, and can put it right back to use as speed again if you choose just by pointing 'em back downhill. I use this feature often when teaching or skiing with a group, whenever I get too far ahead on a cat-track, for example. I just ski up the side and let gravity stop me high above the cat track and wait for the others. When they get to me, I just slide back down again, quickly converting the potential energy I gained by "going uphill" back into the kinetic energy of speed. If I had braked and skidded to a stop on the cat-track, on the other hand, I'd have had to skate and push to get back up to speed. I'd have wasted energy, rather than conserving it.
Anyway, the technical argument aside, the really significant difference between the two is the polar opposite intents they involve. One is offensive (GO that way); the other is defensive (STOP going this way). One leads to gliding (not necessarily pure carving), the other describes skidding. One is a "GO" thought; the other is a "Slow down" or even "Stop" thought.
And these are the intents that define the essence of how different people ski. Great skiers, as a habit, are offensive, gliding, conserving energy, using technique to control direction, and tactics to control speed--they ski "the slow line fast"--"a slow enough line as fast as they can (when they can--and brake when they have to)." Most skiers, from beginner to advanced, and especially the plateaued, stuck-in-a-rut, "terminal intermediates," are defensive, intentionally dissipating energy, using technique to control speed (and, as a natural outcome of excessive skidding, giving up a lot of directional control as a result). They "turn" to control speed, meaning that their "turns" would be better called "braking." They ski "too fast" a line slowly--with the brakes on. And this is true no matter how fast they choose to go. "Defensive" is not at all the same as "timid," and some of the most aggressive skiers on the most challenging runs are still defensive--braking aggressively down the hill.
Intent dictates technique. As I said, these basic, polar opposite intents, more than any other factor, define how individuals ski. Changing from one to the other as a habit is far more difficult than most might imagine, but the rewards are well worth the effort. If you think of your turns as primarily done to control speed, becoming an offensive skier will involve a true paradigm shift. It's not just new information (everyone already knows that going uphill will slow you down, but so few actually demonstrate that they knew it with their skiing). It's not a new thought--it's a fundamentally new and different way of thinking.
These are ideas that we have tossed around here at EpicSki many times over the years, and they ring as true today as ever. The vast majority of skiers are defensive, although few realize it. Think about it. Whether you are a beginner, intermediate, or advanced skier, it could transform your skiing this season!