Plug-in Files Extensions - What are plug-in files and how do I open a plug-in file?

What are plug-in files and how do I open a plug-in file?

File Type Category File Format
.accda Plug-in Files Access Add-in File
.dpm Plug-in Files Pro Tools Plugin File
.ecf Plug-in Files Outlook Add-in
.fxp Plug-in Files VST Audio Plug-in
.mmip Plug-in Files MediaMonkey Installation Package
.nbm Plug-in Files NetBeans Module
.pim Plug-in Files Pro Tools Controller Plug-in Mappings File
.vdf Plug-in Files VirtualDub Video Filter
.vnt Plug-in Files Mobile Phone vNote File
.xpi Plug-in Files Mozilla Installer Package
.xpt Plug-in Files Mozilla Firefox Component