The Plug-In is a locally installed app that will works in the Outlook Client
The Add-In is an app that is enabled via the Office 365 Manage Add-Ins.
The v1 version of the Plug-In and Add-In do not integrate with Salesforce.
The v2 version of the Plug-In and Add-In both have the ability to track shares via Salesforce
Additional Trainings
How to Add Brainshark to Outlook