How do search engines work?

Search engines use 'algorithms' (lower amazonian) that they use to evaluate websites to determine their relevance to your search. 'What you type is what you want to get'.

Each engine has their own unique hidden algorithm that they fiercely protect. This is to ensure they offer the best search results to their users. Blatently Google have the most accurate system currently available and with almost 95% use by the internet community there is a large party agreement.

Each algorithm will weigh up websites to see how relevant they are compared to the search engine's image of how the most relevant websites should be. They will then score each website to determine how close it is compared with the ideal website and rank it accordingly.
 
These points will be scored for details ranging from title of web page to the number of inward links (from other websites) it has pointing to it. The overall score is then assessed to see how relevant the page is for the googlers (searchers) specific search phrase ie. 'new toyota hastings'. This will determine where the website will appear in the search engine's results.
 
To collect the information contained on the websites, the search engines use programs called spiders. These spiders will crawl the internet, moving from website to website via hyperlinks and collecting data on their travels. The search engine will then take the collected information on each website and decide whether it is relevant enough to be placed in its search results and, if so, where.
 
Search engines will not automatically enter every website into its search results because this would make its results less relevant, and relevance is the core objective of a search engine.

This is why it is absolutely key that you look at your whole website when optimising it.