Most people think when they go to Google they are searching the web. But they aren't. The web is a great big place and it's all interconnected, but it isn't organized. What you are actually searching is Google's index of the web. Do you remember the days when libraries had a card catalogue? That catalogue is the equivalent of the Google index. The catalogue is organized and once you find the entry for the content you are looking for, the card tells you where to go find it.
So, what does that actually mean? It basically means that Google "spiders" are crawling the web, clicking on links to see where they go and keeping track of what they find. They try to determine what each website is about by looking at keywords on the site and they store the information. They repeat the process until they presumably hit every website that is connected to another. This is going on constantly, building an index so that when a user searches for a term, Google knows the best results to send back. So, in order to have your website rank within Google, you need to get into the index and you need to make sure that the right keywords get stored.
The only way Google can find a website is if it is somehow connected to another. Or if the site advertises itself to Google by submitting a site map. A site map is really the best way to accomplish this because as your site changes and grows, the site map will make Google aware.