In Microsoft’s words,
“Add-ins are programs that add optional commands and features to
Microsoft Excel.”. Often, these take the form of macros created in a
spreadsheet which are then saved in the “add-in” format, and in the users
add-in directory (usually this is "\
Typically, within the spreadsheet where the macro was created, it would be attached to a toolbar which would be displayed with any other toolbars at the top of the screen. Once clicked, the macro would operate against whatever spreadsheet, worksheet and cell that was active.
Excel already comes with several optional add-ins courtesy of Microsoft. One example is the “Analysis ToolPak” which “Provides functions and interfaces for financial and scientific data analysis”. Further information regarding this add-in can be found at Microsoft’s web site.