This is a “rectangle vs square” situation. Every extension is a plugin, but not every plugin is an extension.
Plugin is the (older) generic term for Ruby code that SketchUp automatically loads as it starts.
Extension is the (newer) more specific term for a plugin that is structured in a particular way using some classes and mechanisms that were created long after plugins were added to SketchUp.
These mechanisms help SketchUp to manage the plugin code. For example, SketchUp can enable and disable extensions without any changes to the files involved. It can embed security signatures and loading policies that reduce the risk of tampering. An extension has author and similar information that can be presented in the SketchUp Extension Manager. All plugins from the Trimble Extension Warehouse are extensions (required for approval there).
For simple plugins, the equivalent (again older) technique to disable a plugin is to rename the Ruby file to something that the SketchUp loader will not recognize as valid Ruby code, e.g by changing the file extension to .rb!. A generic plugin has none of the extra information used by the Extension Manager.