Search engines use complex mathematical algorithms to guess which websites a user seeks. In this diagram, if each bubble represents a website, programs sometimes called spiders examine which sites link to which other sites, with arrows representing these links. Websites getting more inbound links, or stronger links, are presumed to be more important and what the user is searching for. In this example, since website B is the recipient of numerous inbound links, it ranks more highly in a web search. And the links "carry through", such that website C, even though it only has one inbound link, has an inbound link from a highly popular site (B) while site E does not. Note: Percentages are rounded.
Remember when you used to rely solely on search engines for traffic? Remember when you worked on SEO and lived and died by your placement in Google? Were you #1? Assured success. Well, okay, maybe not assured. Success only came if the keywords were relevant to your site users, but it was the only real roadmap to generating site traffic and revenue.