Brennan-
WSM (Winchester Short Magnum) is a fat, short rifle cartridge family.
There are several other families, e.g. the .30-06 brass with a resized neck is used for .25-06 Rem, .270 Win, .280 Rem, .338-06, .35 Whelen, etc. The .308 Win case is the basis for the .243 Win, the .260 Rem, the 7mm-08 Rem, etc.
The difference is that Winchester created the family deliberately, whereas the others were the result of amateur and professional experiments with existing cases that had been adopted by the military so they were available cheap.
WSSM is another family Winchester created, even shorter.
In general, they're short, fat rounds so you can have a shorter rifle action (like a .308) but more velocity (like a .300 Win Mag, or even approaching the mighty but bulky Weatherby Magnum family). The .30-06 and its shortened successor the .308 Winchester / 7.62x51 NATO were developed for the steel available at the time. Modern manufacturing allows magnum-power cartridges in shorter cases than 100 years ago when the .30-06 was developed, or 60 years ago when Roy Weatherby began playing with hand cannons.
Remington has their own competitor, the SAUM or Short Action Ultra Magnum. In keeping with Remington's more conservative approach, they make only a few calibers.
A bit of history: Winchester seems to use a "shotgun" approach to cartridge introduction. They put a bunch out there and some of them hit the mark. Remington seems to use a more targeted approach. Both companies have introduced many successful cartridges over the years. I believe (someone correct me if I'm wrong) that while there are some rare but cult-beloved Remingtons like the .280, there are a lot more obsolete, orphaned rounds out there with the suffix "Winchester" than "Remington". That's probably one reason why people ask the question about a newfangled Winchester innovation's future in the marketplace.
The .300 and .270 WSM's seem to have hit critical mass. I don't know about any of the others.