Search engines utilize indexing software agents often called robots or spiders.
They will essentially go from URL to URL until they have visited every Web site on the Internet. It will then continue on to visit all external links. Following these external links is how search engines are able to find your site regardless of whether or not you register your URL with them. Submitting your URL, however, does speed up the process. It notifies an agent to visit and index your site instead of waiting for it to eventually locate you through one of your external links.