A Word add-in is installed functionality that adds custom commands and new features to Microsoft Word.
Word add-ins are self-contained programs that modify, extend or add functionality to Microsoft Word desktop client. Currently, there are three different types of add-ins.
Web add-ins use the Office JS (Javascript) APIs introduced in Office 2013 and extended in Office 2016 and later. They are platform-independent, running on the Windows desktop, on the Mac, in Word On-line and in Word for the iPad. Currently the APIs do not support the full spectrum of Word functionality provided by the VBA/COM object model. Priority is given to functionality supported across all platforms.
COM add-ins and VSTO add-ins (VSTO Add-ins base on and extend the COM IDTExensibility2 interface) must be registered under Windows. They are loaded when Word starts and can be set to load automatically or on-demand. They work with the same object model as VBA; VSTO being .NET code accesses the objects via the PIAs (Primary Interop Assemblies).
Template add-ins are Word templates containing VBA code, Ribbon customizations and/or keyboard shortcut assignments. Placed in the designated "Start-up" directory (folder) they will be loaded automatically when Word starts and their customizations will be available to the user. Template add-ins can also be loaded and unloaded manually or explicitly through other code.
