Watch the video embedded below or it can be seen at msdn.com. The first fifteen minutes or so is just fluff about how much money you can make writing Windows 10 apps.
Microsoft seems to be genuinely unifying the mobile and desktop Operating Systems. The irony is not lost on me that Microsoft is doing the thing everyone said Apple would do. For years peoople have claimed Apple would turn OSX into IOS but they have done nothing in that direction. With Microsoft's new unified OS strategy, you code to one API that will produce one binary that will be able to run on all platforms: desktop, mobile, and xbox. There will be some specific to each platform API elements you can call but you don't have to.
The adaptive UX development kit allows you to create a user interface that will be responsive (As a web developer I hate that term) to different devices. You can have controls, app elements, menus etc change the way the are represented based on screen or windows size. You can also change interfaces based the the platforms capabilities. You can have the interface adjust to whether there the device has a touch screen or perhaps keyboard and mouse.
As a web developer I found the hosted web apps interesting. Short version, you submit your web app, basically your website, to the App store. You can then send them some meta data to request access to certain OS APIs. For example, you can have your web app open outlook and create a calendar event. It reminds me a lot of active x from years gone by. I hope they don't have the plethora of security problems they had with active x.
I gotta say Microsoft is doing a lot of forward thinking and exciting things these days.