One of the greatest things about WordPress plugins and themes is the ability to add shortcodes to do something fancy to your post content. Introduced in version 2.5, shortcodes are a simple set of functions for creating macro codes for use in post content. The most common of these is of course the gallery shortcode.
It enables plugin developers to create special kinds of content (e.g. forms, content generators) that users can attach to certain pages by adding the corresponding shortcode into the page text.
— WordPress Codex on the shortcode API