A search engine needs to store and access information about your website in order for people to find it when they search the Web. If the search engine does not have the necessary information, it cannot return links to your site, resulting in less traffic and fewer potential customers.
It is simple to use a search engine to find links to pages containing information you are searching for, but what seems simple in this case, is actually quite complex.
Suppose a person types "organic farming" into the search box of a search engine, and hits return. The search engine now wants to find a site with "organic farming" content and, at the same time, it does not want to return unpopular sites. If the search engine were to list the unpopular sites people would not be satisfied with it and would simply use another search engine.
Each day trillions of pieces of new information are added to the Web. For this reason search engines are constantly crawling through the Web using Web crawlers known as spiders to index websites and log content as they travel from website to website via existing links.
A spider uses an algorithmic process: computer programs determine which sites to crawl, how often, and how many pages to retrieve from each site.
There is some content that a search engine cannot find or index by crawling. Videos are a good example in that a search engine cannot watch a video and log content from it. In order to index this content, sitemaps were invented. A Sitemap is a protocol used to tell search engines about Web pages that are available for crawling, essentially enabling the search engine to find the pages on your website that might not have been discovered without a sitemap.
Following the fundamental rules of SEO (Search Engine Optimization) when building your website, will ensure that you are off to a solid start. Correctly implementing a sitemap will further aid in getting the content that normally would not be indexed by a spider, indexed. This will allow search engines to find more content from your website and thus allow more searches to list your site as a result, hopefully on the first page.