Notepad++ is very extensible using so called plugins. Plugins are small or big additions to Notpead++ to enhance its functionality. Notepad++ comes bundled with a few plugins (when using the installer, you can choose which ones to add), but you can always add your own or remove some. The plugins are located in the Plugins directory in the main Notepad++ installation directory. They are DLL files and simply removing or adding them is enough.
Be careful to make sure your version of Notepad++ is up-to-date enough for new plugins and that the plugin is compatible with Notepad++ (some very old plugins will not work with newer versions of Notepad++, it is up to the developer of the plugin to add support or not).
Currently, Notepad++ can be found in an (older) ANSI version and the newer Unicode verison. The plugin has to match this version, otherwise it will not work (Notepad++ will warn you about this). Plugins can be found anywhere on the internet, but the “official”collection can be found in the Notepad++ Plugins project on SourceForge.net: https://sourceforge.net/projects/npp-plugins/