A Plugin is trigger whenever a page or post loaded .
A shortcode is the code within a  braces which will trigger a functionality into front end . When a page or post loaded or requested , plugin manager read the post content and if it found any shortcode which is register for any plugin installed and will trigger the specified functionality .
Shortcode for a plugin is a very important functionality and hook of in a plugin development for display data in front site.
Example Of Shortcode : [shortcodename attr1=value1 attr2=value2 .......]
WordPress Hook for shortcode :
function callbackfunction ($atts)
include ‘shortcode.php’; // Functionality file if you want to code in separate file
Make sure used Shortcode name is not using by any of the installed plugin in your wordpress .