After you’ve installed WordPress you’ll discover a world of plugins, each designed to increase your website productivity.
WordPress plugins are add-ons for WordPress that plug into the core WordPress application and add extended functionality to the tool. Plugins can range from visual changes such as my simple Smiley Remover to complex reporting tools such as Google Bot Bling. What each plugin has in common is that they add to the power of the WordPress platform by either replacing or adding features.
There are a few core plugins which are developed by the team who puts WordPress together but most are written by third party developers and while many are free, some are paid or premium plugins. The WordPress team does a great job of maintaining a list of nearly 11,000 plugins at http://wordpress.org/extend/plugins/ and each plugin is ranked or rated according to community usefulness. A list of the most popular WordPress plugins can be found online (including one of mine) at http://wordpress.org/extend/plugins/browse/popular/.