VSO Extensions is the new UI extension model that is being used for web access. It allows us to create custom supported extensions throughout the web access. This includes extending areas such as menus and toolbars into the following hubs:
Build
Code
Test
Work
You can also create your own hubs and hub groups if you want, and then build a complete app UI embedded inside TFS for applications such as the TFS time tracker (http://www.tfs-timetracker.com/).
VSO Extensions is, as a single package, called an extension, and then inside each extension are one or more contributions. These contributions can be all the core TFS contributions, or they can be contributions exposed from your own extension or other extensions.
The extensions are mostly HTML and JavaScript with an extension manifest file that has metadata about the extension presented as JSON. For security purposes, the content for these extensions needs to be hosted using HTTPS. This isn't a problem for small and community...