Search engine optimisation is one of those industries that works behind the scenes. The kind of business that the vast majority of people have no idea exists, but that effects their lives on a daily basis.

Whilst nearly everyone will have used Google at some point - billions of searches are made each day by millions of unique users at some point - very few people will have any idea about exactly how the search engine gives them relevant results. If they do, then they might actually have some idea about just what search engine optimisation is and how it works.

Still, even these people would probably be shocked at the size of the industry and the passion of the little community that has sprung up around it. Known internally as SEO, it's a complicated little world of technical intricacy and constant debate that the average outsider will find slightly incomprehensible.

If you're still wondering what search engine optimisation is, prepare for the info-dump. SEO is all about improving the volume or quality of we traffic for a web page - whether it's an ecommerce site, a blog or an online gallery - through one specific source: search engines. Whilst it's an internet marketing strategy, it's not commonly thought of as "search engine marketing.'' This is because SEO stands separate to buying advertisements in search engines; ultimately, it's about making a site appear higher in search rankings.

This is actually incredibly important for two reasons. One is that if your website doesn't appear in search, then just what are the chances that anyone will visit it? Relying entirely upon direct marketing can be very expensive but risks delivering little to no traffic to your website if it doesn't reach the right people. Secondly - and most important of all - even if your website is in Google's index, if it doesn't appear in the first 20 results then 90% of web users will never see it.

Ultimately, SEO consultants offer to help you reach this 90% of users, earning a site both increased visibility and the knock-on reputation boost of being recognised by Google as the "most relevant" page for that query. They offer to make your site and your services "search friendly," targeting certain keywords that your prospective audience is likely to type in and "optimising" your site at these terms.

The way it works is that big search engines use programmes called 'crawlers' to search web pages, storing them in an index and ranking them according to their internal algorithms. These algorithms are used every time someone enters a search query, calculating the most relevant page for the words entered. This is complicated by the fact that the exact nature of this algorithm is kept under wraps - and it's incredibly complex.

Search engines also use location, the time the content was added, the time the search was made, the user's search history, meta-information (descriptions embedded in HTML code), site structure - no one knows the full list. Working out just what these factors are and how to best use them forms the bulk of discussion between the members of the SEO community, who are constantly engaged in prying secrets from companies like Google and working out just how to capitalise on them.

The biggest areas of work for SEO though are in optimising content for relevant keywords and increasing the "link reputation" of a site. One of the biggest factors in search relevancy is thought to be how many links each site has from other websites; the theory goes that if people are pointing others to your site, you must be of some relevance to somebody.

Although this has given a broad overview of SEO, it's barely scratched the surface of the industry. Hopefully if you've never heard of it before you'll have some idea about what it is and how it happens.

Next time you search for something on the net, keep in mind just how many sites each engine returns and ask yourself - why is this at #1?

The chances are (unless it's Wikipedia - a site that is an apt lesson in how SEO can come from spontaneous user activity) that they've got SEO at every level of their site.