A plug-in is a piece of third-party code that extends the capabilities of a website. It is most often used in conjunction with a CMS or blogging platform. Plug-ins are a way to extend the functionality of a website without having to re-write the core code of a website. Plugins can also refer to pieces of third-party software installed within a computer program to increase its functionality. The Firefox web browser has an extensive catalogue of Plugins (called ‘Addons’), which are used to extend its functionality.