Your scripts can remain on your own device or account, or you can publish them for others to download, run and modify. You can even write full apps for publishing in the Windows store.
The list of supported browsers makes interesting reading: Obviously IE10 is there so you can code on your shiny new Surface, but iOS, Android and Chrome browsers? Write code on your iPad or Chromebook but for Microsoft products!
Not all APIs are yet implemented in the web app, it is only a preview at the moment, but it's free to use and has examples and tutorials that make it easy to get started with