WinJS 2.0 was introduced with Windows 8.1 with some additional UI controls. As well as being used to create WinRT apps, it was also extended to Xbox including look and feel.
WinJS 2.1 was announced at Build and this extends its reach yet again to include Windows Phone 8.1, also launched at Build.
In other words, WinJS now works with the two newish Windows APIs - WinRT and Windows Phone.
Going beyond Windows, however, seems to be the aim of the open source project. WinJS will be usable on Chrome, Firefox and IE, if the goals of the project are met. What this means is that you can have a single Web app that runs on WinRT, Windows Phone and in a standard browser and it should look the same on all platforms.
Of course the downside is that this is yet another Web App specification to add to Chrome apps and Mozilla's Open Web Apps. At the moment none of the specifications is well enough implemented to win out over the others, so perhaps WinJS is in with a chance to be more than just another UI library - only time will tell.
The new WinJS website is also nice in that it gives you the chance to try WinJS in an interactive editor. If you are curious about what WinJS is all about then try it out - it is the easiest way to see the range of UI controls, for example.
If you want the code or if you want to contribute then you will find WinJS on GitHub under an Apache 2.0 license.