There are many wordpress plugins which are using the shortcode.
Since Version 2.5 WordPress support so called Shortcodes. They have been introduced for creating macros to be use in a posts content. Many people looking for how to create the shortcode in wordpress using theme. It is very easy to build shortcode in wordpress. I given very simple code sample for creating the custom shortcode.
For more information visit following page.
For creating the custom shortcodes you need add following method in functions.php file which is located in your theme folder.
return '<img src="http://images.wordpressapi.com/How-to-make-empty-the-wordpress-trash-automatically.png">';
How to create custom shortcode for your wordpress site
you can change the return text or shortcode name aslo.