When you are editing a post you can notice the URL in your browser looks like
http://***.com/wp-admin/post.php?post=1351&action=edit
The number in the URL corresponds to the ID of your post. So you can access your post with that ID. In this example the short link for the post would be
http://***.com/?p=1351
When a visitor browsers this URL she will be redirected to the full URL if you have the correct options set in Settings > Permalinks.
If you need to get the short URL programatically you can use the wp_get_shortlink() function. This function returns the link, which can then be printed.