For the new Add-ins for Office please also add the office-js tag to your question. Microsoft Visual Studio provides project templates you can use to create application-level add-ins for Microsoft Office 2010 and Office 2007. You can use add-ins to automate Office, extend Office features, or customize the Office user interface.
Add-ins for Office are a new way to create extensibility solutions for Microsoft Office. These Add-ins can be run as a taskpane on the side or inside the document dependent on the application.
Visual Studio Tools for Office (VSTO) is a set of development tools that allows the development of Office automation solutions (add-in or document customization), using .NET and the Visual Studio IDE.
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