What is search engine optimisation (SEO)?

Search engine optimisation (SEO) refers to methods for designing your website to increase the likelihood of search engines finding it and giving it a prominent rank in their search results pages.
If your website is ranked within the first couple of search results, you are more likely to attract visitors to your site than if your website appears on the second page of results.
SEO refers to improved positioning in ‘organic’ search results. That is, search results based on the content and related information about your site as opposed to paid advertising (see search engine marketing).
The core purpose of a search engine is to provide the most relevant results in response to a search term. The methods that search engines use to rank websites in response to search terms are not generally made public.