Microsoft Visual Studio Team Services is Team Foundation Server (TFS) on the cloud, rather than on-premises. Formerly it was known as Visual Studio Online and even before as Team Foundation Service.

It provides the following ways of extensibility (integration):

• JSON REST APIs: you can get or modify information of a Visual Studio Online account (team projects, users, commits, work items, etc.)
• Service Hooks: they provide the ability for your app or service to get notified instantly when an event happens in Visual Studio Online. Service hooks let your app or service to avoid continuously polling to check for new resources, like completed builds.
• Extensions: extensions enable you to create first-class integration experiences within Visual Studio Team Services, just the way you have always wanted. An extension can be a simple context menu or toolbar action or it can be a complex and powerful custom user experience that lights up along side Microsoft-provided capabilities. Users can find your extension in the Visual Studio Team Services Marketplace, install it, and use it with their Visual Studio Team Services accounts.