Background: A web browser plugin is a software program that adds extra
capabilities to your web browser (such as the ability to view movies, see
Flash animations or run Java applets).
Plugins allow third-party developers to extend a browser's capabilities;
to support features that might have been unforeseen when the browser was
developed; to reduce the size of a browser; and (most commonly) to separate
the plugin source code from the browser source code because of software licensing
issues.
Plugins began to be invented not long after browsers first appeared. All
major browsers support them. And these days there are two plugins that
are absolutely essential for everyone who uses the Net - Acrobat
Reader (to read PDF files) and Flash Player (to view Flash
animations).
The downside of plugins is that since they run with all the privileges
of real applications, they can do absolutely anything to your computer. And
this weakness has been exploited by many corrupt companies over the years.
You can avoid this risk by never, ever agreeing to install a plug-in
unless you have very good reason to trust the source. This is why most people
now limit their plugins to a handful of essential ones from the world's leading
IT&T companies (eg: Adobe, Apple, Google etc).
The upside of plugins, though, is that they're all absolutely
free. And here's where you can get the main ones:
Browser Plugins