Checkout - wp-includes/plugin.php#L585
plugin_dir_url() function internally uses
plugins_url() to get the link to plugin directory.
This will return url of the plugin directory with a trailing slash at the end. So this can be easily used to link to the plugin directory.
If no arguments are passed this will deliver the same result as the above function; but with or without a trailing slash at the end. This can be configured to link to files within plugin directory; a useful shortcut.
plugins_url( 'img/bar.jpg' , __FILE__ ) will return a url like