I'll probably get roasted for this, but I can't help but think that "UWP" is the latest in a long line of Xbox buzz phrases that gets prematurely latched onto by fans. I certainly see what Microsoft's trying to do with it, but I'm not quite sure it's going to pan out quite as perfectly as posters in this thread anticipate.
Again, we'll see. Interesting times ahead.
UWP is a Microsoft thing as a whole. It's essentially their new container that they want to replace WIN32 with eventually. It relates to Xbox in the idea that it is supposed to aid in maintaining compatibility across platforms as new hardware comes out as well as allow them to unify their stores to make those more cohesive. It's going to be a few years before they get to something that is mostly acceptable for PC users since they are still locked down in a kinda shitty way.