Wordpress plugins are small customised programs that can extend it to do just about anything you can imagine. There are thousands of plugins and they are made by all sorts of people. On the one hand it has made web development with wordpress exciting and very flexible but on the other hand it has made it vulnerable, as plugins can sometimes have bugs in them that clash with other plugins or versions of wordpress hence causing you headaches.
It is tempting to go mad with plugins, however it is wise to be prudent and only select what is necessary. A well designed theme wont need a lot of extra plugins which avoids problems when your version of wordpress needs updating. New versions are being released frequently and it is recommended that you backup your entire website and disable all plugins before updating wordpress, then switch them back on one at a time to make sure they are all still working alright with the new version.
There are over twenty five thousand wordpress plugins so selecting the right one for the job is often a painstaking process. You might be looking for an SEO plugin to help you optimise your pages or a photo gallery plugin to exhibit photographs on your website. Just as their are premium wordpress themes that you can buy, the same applies to plugins. Some of the free plugins have a pro version that you pay for to get additional features. There are some excellent paid premium plugins available here Code Canyon