HostGator.com will NOT place ads on your site. That is one of the many advantages of having a paid host.
We do have a default 404 page that can be replaced in a few seconds with your own. We recommend doing this to help maximize your traffic. A 404 page is what loads when someone visits a page on your site that does not exist. If you aren't sure how to apply your custom 404 page to your website, please visit our page on custom error pages.