UWP is ambiguously defined and the acronym's usage is on the way out. An evolution of UAP and Windows Store apps, it boiled down to a whitelist of APIs allowed on multiple devices (the U in Universal) and encompassed Win32, WinRT and COM APIs. UWP itself isn't an entity with APIs, is not a technology, and the original concept is dead thanks to new platform leadership at Microsoft. (Still in play on Hololens, Surface Hub, and other sandboxed devices for now...)

That said, "UWP" is confusingly also used to refer to Windows apps that tap into WinRT APIs, participate in the modern app lifecycle, are modeled and have identity, or generally tap into newer technologies such as AppContainer. You'll see this, for example, perpetuated in Visual Studio in its project templating. It's a BIG OL' MESS right now; I'm working with/pushing Microsoft to get this communicated clearly and mopped up but old habits die hard.