A plugin is a small program written by a third party that can be integrated into an application for increased functionality. In a nutshell: plugins turn your blog from something ordinary into something sophisticated and optimized for maximum traffic.
There are over 2,000 plugins available for Wordpress that allow you to do everything from managing spam, creating an events calendar, improving tags, viewing statistics…and much more.
You can find the full list of available plugins at http://wordpress.org/extend/plugins/
One of our favorite plugins: The All in One SEO pack
Automatically helps you to avoid duplicate content found on typical Wordpress blogs.
Automatically optimizes your titles for search engines
Allows you to override any title and set any META description and any META keywords you want.
Automatically generates META tags
Allows you to fine tune anything you want.
Is backwards-compatible with a lot of other plugins, such as Ultimate Tag Warrior, Auto Meta,...