Most search engines such as Google work in a similar way. They send out automated robots, referred to as 'spiders', to build a picture of the Internet. 'Crawling' or 'spidering' enables a search engine to build a list of domain names to build its search index. This happens periodically to ensure listings are up to date.
In order to make your domain visible to search engines, you can either submit it directly or you can rely on it being found from links on other websites.
There are other considerations as some search engines also use others search engines and other directories, such as the Open Directory Project (DMOZ), in addition to their own spidering.
Search engines use highly complex algorithms to determine how high up a site will be listed for a given keyword or phrase. The key message nowadays is 'don't try to cheat!'. Search engines constantly change and adapt the algorithms they use to try to prevent people from cheating the system. By approaching search engine optimisation (SEO) in an organic (and honest!) way, you can make your site perform extremely well in search engines and contribute to the building of a genuinely useful index of the Internet for all of us.
There are three main concepts to understand:
In search engine parlance, these are referred to as indexability, relevance and link popularity. The first two are affected by the way the site is built and the way the content is added to it. The third one is more about providing content that people want to link to.
Indexability is largely a technical concept and needs careful consideration when the site is built. Essentially, it refers to how well a search engine can index the site once it has found the site. For instance, if all of your navigation relied on JavaScript or Flash, you will have a problem!
Relevance is a much more elusive concept. Looking at relevance from a high level, it means how relevant your site is for a particular key word or phrase. Deciding which keywords to use and where is the tricky bit. The easier part is following the rules once you've decided upon these. Search engines will weigh up many things in deciding how to score your website for relevance. These include: page title, meta tag description, meta tag keywords, headings (H1, H2, H3, etc.), alt text, keyword placement and density.
You must approach optimisation of the site on an individual page basis. Having the same page titles and meta tags on all pages will not help - try to make these relevant to the content of the individual page. The key with relevance is not to try to cheat the system; you'll only end up cheating yourself!
In broad terms, link popularity is measured by the number and importance of the sites that link to your site. Search engines will measure the quantity of inbound links and weight these according to the perceived importance of the linking site. For instance, one link from the BBC website is likely to be deemed more important than the cumulative worth of ten links from personal websites.
Link popularity is the main contributing factor in determining Google's PageRank.
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