Plugins are add-ons to your wordpress blog. They extend the functionality of your site. There are thousands of plugins for wordpress, they are free and most of them are upgraded regularly. They can also be easily switched off. This makes wordpress very convenient, flexible and easy-to-customize software.
Plugins are software scripts. Plugin is usually a number of files compressed into archive that can be downloaded to your hard disk from plugin directories or from the site of the plugin. This is the official plugin directory: http://wordpress.org/extend/plugins/ You don’t need to have programming skills when using most of the plugins but you have to read carefully the instructions given by the plugin author and follow them. If something is wrong with the plugin you should consult the plugin author. When selecting a plugin you should see to compatibility of the plugin with the version of your wordpress blog. Very often it is written in the description of the plugin. It is strongly recommended to make backup of your site in case something go wrong, before installing a plugin.