From SC4 Encyclopaedia
A plugin is a file or set of files that is stored in one of the Plugin Directories and loaded into the game at startup. A plugin is always a DBPF (except in the case of the Extra Cheats dll and the background PNGs). It alters the game in some way, maybe adding a growable building, maybe changing the speed of the rail network. Below is a list of every Plugin documented on the SC4 Encyclopedia, and thus has an article for it, and a list of all plugin subcategories.