Many software applications have been developed with the ability for outside developers to add their own extra features to the main software. This extra software is called a 'plugin'.
For example, Photoshop has a thriving community of plug-in developers. They make fancy filter effects, image adjustment tools and so on.
The web browser 'Firefox' also has many people developing useful plug-ins that improve on what the basic Firefox application can do. In this case the plug-ins are called add-ons (see https://addons.mozilla.org/en-US/firefox/)
Some browser plug-ins are so useful that most people need to install them if they are to get the best out of the web - for example the 'Flash Player' plugin from Adobe is needed to view Flash movies.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Plugin