Add-ins are supplemental programs that extend the capabilities of Microsoft Expression Web by adding custom commands and specialized features. You can obtain add-ins from third-party vendors or you can write your own custom add-in programs by using an external compiler, such as Microsoft Visual Studio.
To conserve memory and increase the speed of Expression Web, it's a good idea to remove add-in programs you don't use often. When you remove an add-in, its features and commands are removed from Expression Web, but the add-in file itself remains on your computer so you can easily add it back in when you need it.
Add-ins are not the same as plug-ins. A plug-in is one of a set of software modules that integrate into web browsers to offer a range of interactive and multimedia capabilities.
Code written in add-ins could access your computer files and network connections outside the context of Expression Web. Make sure your add-ins come from reliable sources.