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 plugin directory with a trainling slash at the end. So this can be easily used to link to plugin directory.
If no arguments are passed this will results same as of above function, but with or without trailing slash at end. This can be configured to link to files within plugin directory.
plugins_url( 'img/bar.jpg' , __FILE__ ) will return a url like