What is the difference between Wpdb and WP_Query?

What is the difference between Wpdb and WP_Query?

$wpdb is a PHP global variable that holds the WordPress database object, which is actually an instantiation of the wpdb class. WP_Query actually uses wpdb to query the database. Use WP_Query when you manipulate native WordPress tables (the vast majority of common tasks). Use $wpdb when you manipulate custom tables