The update URL is a URL on your server. The URL will be used to indicate that the status of one of your orders has changed. If a status has changed, our system will call your update URL on your server. We'll send a notification (without status info). You'll then be able to retrieve the new status.
Return URLs are URLs on your website. They are used to redirect your customer to inform him about the state of the order. There are 4 Return URLs: Cancelled, Error, Pending en Success.