The standard internet browser (the program that reads internet files) such as Netscape, Internet Explorer, Opera, and Konqueror are built to read certain file types such as webpages which are .html .htm and .shtml files, as well as certain types of graphic files such as .gif .jpg and .png (which is the replacement format for the .gif).
Plugins are a mechanism to allow your browser to handle (read, play and display) other types of files. Each new version of the browsers released adds capacity for the browser to handle extra file types internally, but plugins are still required for most multimedia (sound, video etc.) files. So if you open a webpage that has music, a video, or a recorded voice-over, then a plugin will be used to play it.
There is a defined set of rules for plugins and any piece of code (programming) that works to those rules can communicate with the browser. When the plugin is installed it automatically tells the browser about itself, including detailing a list of the file types that it can handle. So if you open a webpage which has a sound track your browser will look up an internal list of the plugins already on your machine and will activate the first one it finds that says it can handle the sound file in question.
If your machine doesn’t have a plugin to play the music file, a window will usually pop up telling you that you do not have the plugin to handle this file and offering to take you to the appropriate site to download one that will. Sites which offer plugins usually make it easy to find and download the plugin required in a one step operation, but BEWARE, there are serious implications in downloading software of this nature, as bogus plugins may damage your computer, so only download plugins from trusted sites, such as:
Video files may also present a problem with their encoding format and as such many will required an additional mechanism to assist your browser to handle them called a codec. These are a sort of plugin for a plugin. The .avi format is one that may use a variety of codecs, so don’t be surprised if you have loaded a video plugin, have been happily viewing vidoes, and are suddenly told you need a codec. Many sites that incorporate video files will provide a link to enable you to obtain the codec they use but again it is always safer to obtain these from a known and reputable source, as the same security considerations exist as for plugins, or in fact for any other software, you download.