Plugin
A plugin is a type of functionality that can be turned on or off as needed, as long as the website it is activated on uses a compatible theme. Plugins give you access to common features across multiple websites. Some examples of plugins are BU Banners and BU Landing Pages. They are designed so that if you redesign your site, as long as the new site is compatible and these plugins are still actively developed, you will not lose your content. Because of the way plugins work, they tend to be able to share features across multiple sites, while in the case of themes, features are generally limited to that theme, are very specifically built for that website's use case only, and are difficult to modify or add to a different website's theme.