The Shortcode API is a simple set of functions for creating WordPress shortcodes for use in posts and pages. For instance, the following shortcode (in the body of a post or page) would add a photo gallery of images attached to that post or page: [gallery]

The API 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.

via codex.wordpress.org