How To Create A WordPress Plugin - Smashing WordPress
►http://wp.smashingmagazine.com/2011/09/30/how-to-create-a-wordpress-plugin
WordPress plugins are PHP scripts that alter your website. The changes could be anything from the simplest tweak in the header to a more drastic makeover (such as changing how log-ins work, triggering emails to be sent, and much more).
Whereas themes modify the look of your website, plugins change how it functions. With plugins, you can create custom post types, add new tables to your database to track popular articles, automatically link your contents folder to a “CDN” server such as Amazon S3… you get the picture.