how do I know a form has been submitted?

when someone fills out a form on-line - is there some notification received so I know to to review it?

Yes, you will receive email notifications. Every form has its default notifications. To find out what email recipient is used, follow this guide: http://www.jotform.com/help/41-Finding-out-E-mail-Address-used-for-submission-notifications

You can also view the submissions in your form submissions page http://www.jotform.com/help/269-How-to-view-Submissions