Notice I'm not going to talk about tuning skis. Waxing is part of the tuning process, but waxing is not tuning. Make sense? If it did, skip ahead. Otherwise, tuning is essentially the complete process of resurfacing the edges and bases of skis, including sharpening and beveling the edges, repairing damage to the bases and/or edges, stonegrinding the bases in order to give them structure, and finally waxing and finishing the bases.
I used to try to do edges and bases myself, but I decided that other than knocking burrs off with a diamond file, I'd just as soon pay a shop $30 or so to do it since stonegrinding is an important part of that process and I don't have a wintersteiger machine in my garage. I usually get this done once a year, and that seems to be adequate.
Stonegrinding ski bases provides the base with structure, which is just a fancy way of saying it makes it so the bases aren't perfectly smooth. Base structure is the slight but consistent pattern of irregularity that enhances a ski's ability to slide across the snow. If skis were perfectly flat, they would probably still slide, but not as well. Think of applying a suction cup to glass versus a piece of ceramic. The surface irregularities keep the skis from sticking to the snow the same way they keep a suction cup from sticking to ceramic.
Wax also aids the ski's ability to slide across the snow. Skis slide on snow because the pressure the skis apply melts the uppermost surface of the snow, providing a thin film of liquid water on which the skis slide. Wax is hydrophobic, so this thin film of water doesn't stick to it, and the ski will slide more easily than it otherwise would. If you've ever stepped on a wet and recently waxed floor, you probably know firsthand what I'm talking about.
If skis are not waxed, they will typically still slide downhill, but a ski's base material is somewhat porous (which is why the wax sticks to the base), and I've seen instances where a ski base that hasn't been waxed will have snow freeze and stick to it. Unwaxed skis will be slower in general and much slower on a traverse or low angle slope.
Aside from enhancing the ski's ability to slide down the hill, wax also enhances the durability of the base, thus extending the life of the ski. Waxed skis are less prone to damage when you hit a rock or a root, so time spent waxing generally means less time or money getting bases repaired.
One would think that waxing a ski would negate the benefits of stonegrinding and having base structure. And if waxing and scraping are all that are done, that is indeed the case. Which is why most ski technicians will wax the ski, scrape the cooled wax, and then brush the ski either by hand or with a machine in order to expose the base structure. I'll get into how I do all this stuff in a subsequent post, which will not happen until I get my garage organized enough to allow access to my bench.
Of course the explanation I've provided is still pretty rudimentary, and it gets much more complex than this, with different wax compounds and materials designed for different snow conditions and temperatures. Cross country skiers and in particular racers, unaided as they are by gravity, take their waxing much more seriously than your typical alpine skier. Alpine racers, for whom races are won and lost by hundredths of seconds are similarly fanatical about their wax.
Though I know some recreational alpine skiers who wax with a temperature-specific wax every time they go out, most skiers don't even wax their skis themselves, and shops almost always apply a general purpose or all-temperature wax like I do. Other (typically but not always Nordic) skiers will put their skis in a hot box in order to get better wax penetration or will apply layers of different waxes in order to take advantage of different properties. Indeed, like anything else, waxing can be quite complex, and I don't pretend to be an expert. So if you know something I don't or I've provided gross misinformation somewhere, leave a comment and let me know. Since my skis are about due for wax, maybe I'll get to organizing the garage sooner rather than later and get on with part 2.