Plugins are tools which provide additional functionality to your application.
To install a plugin you generally just need to put the plugin file into your ‘wp-content/plugins’ directory.
Once a plugin is installed, you may activate it or deactivate it from the Plugins menu in your WP administration.
In WordPress 2.7 and above you can install plugins directly from the admin area.
Download the plugin to your computer, then log in to your WP admin area and go to Plugins > Add New.
Browse to the plugin archive and select it. Then click Install Now and the plugin will be installed shortly.