Plugins are utilities which provide additional functionality to your application. To install a plugin you just need to put the plugin files into the wp-content/plugins directory. Once a plugin is installed, you may activate it or deactivate it from the Plugins menu in your WP administration.
Since WordPress 2.7, 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.
Deactivating plugins is straight forward. Simply navigate to the Plugins menu in your WP administration, find the plugin you want to deactivate and select the Deactivate button.
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.
Still always bear in mind that not all plugins are easy to install, so always refer to their documentation and installation instructions first.