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.
The removal is also straight-forward in most cases. Simply remove the directory for the plugin you would like to uninstall and it will be automatically deactivated.
To add a WordPress Plugin using the built-in plugin installer:
Go to Plugins > Add New.
Find the WordPress Plugin you wish to install.
Click Install Now to install the WordPress Plugin.
The resulting installation screen will list the installation as successful or note any problems during the install.
If successful, click Activate Plugin to activate it, or Return to Plugin Installer for further actions.