A Shared Add-In in an Add-In which is shared between multiple office applications.
Shared Add-Ins unlike VSTO Add-Ins can be written for more than one application i.e can be shared between MS Excel, MS Word etc. Unlike VSTO Add-Ins, Shared Add-Ins has to implement different methods for "connection", "un-connecting", "startup" and "shutdown".
Shared Add-Ins have a different deployment method as compared to VSTO Add-Ins
