What is WP-Cron in WordPress? How is it different from a real cron job?

WP-Cron is the scheduler of WordPress. It does not run all the time like a server cron. It checks for due tasks when a page is loaded (through wp-cron.php), so a task can be late if there are no visitors.

Schedule your own task with wp_schedule_event() and attach a function to its hook:

add_action( 'tc_daily_task', function () {
	update_option( 'tc_last_run', 'task ran' );
} );

if ( ! wp_next_scheduled( 'tc_daily_task' ) ) {
	wp_schedule_event( time() + 3600, 'daily', 'tc_daily_task' );
}
echo 'Scheduled: ' . ( wp_next_scheduled( 'tc_daily_task' ) ? 'yes' : 'no' ) . "\n";

do_action( 'tc_daily_task' );   // run it now, to test
echo get_option( 'tc_last_run' );

Use a real cron for big sites

Turn off the visit based trigger, and call wp-cron.php from the server every few minutes:

// wp-config.php
define( 'DISABLE_WP_CRON', true );

*/5 * * * * cd /var/www/html && wp cron event run --due-now >/dev/null 2>&1

Remember to remove the scheduled event when your plugin is deactivated, with wp_clear_scheduled_hook().

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