Visual Studio Tools for Office

Visual Studio Tools for Office (VSTO) is a set of development tools available in the form of a Visual Studio add-in (project templates) and a runtime that allows Microsoft Office 2003 and later versions of Office applications to host the .NET Framework Common Language Runtime (CLR) to expose their functionality via .NET. This allows extensions to the Office applications to be written in CLI compliant languages as well as to use functionality and user interface constructs from Office applications in .NET applications. Extensions to Office prior to Office 2003 only allowed the creation of COM add-ins using Visual Basic or Visual C++ and a "Developer" edition was also offered that enabled VBA developers to create COM Add-ins.

Source: Wikipedia — Visual Studio Tools for Office (CC BY-SA 4.0)