A shortcode is a WordPress-specific code that lets you do nifty things with very little effort. Shortcodes can embed files or create objects that would normally require lots of complicated, ugly code in just one line. Shortcode = shortcut.
Where I can find Shortcode tool?
In content edition window just click ‘shortcode‘ button, and the list of all shortcodes will be shown.
In WordPress 3.9 you have to change content type to ‘Text’ to see the shortcode button. We’re working on it, to have it also in ‘Visual’ version.
Here you can learn more about shortcodes.