An affiliate program is a marketing program where you can receive commission for helping a parent company (such as HostGator) generate sales. The parent company provides you with all your advertising tools such as banner ads, links and tracking codes for you to place on your sites, in your emails or other internet listings.
When a visitor to your site or listing clicks on an ad or link, the transaction is tracked. If this transaction results in a sale for the parent company, you get paid for that sale. It's that simple!