Google has stated that it uses more than 200 factors or signals within its search ranking algorithms. These include positive ranking factors that help boost position and negative factors or filters which are used to remove search engine spam from the index where SEO companies have used unethical approaches such as automatically creating links to mislead the Google algorithms. The two most important factors for good ranking positions in all main search engines are:
1. Matching between web page copy and the key phrases searched
The main factors to optimise on are keyword density, keyword formatting, keywords in anchor text and the document metadata including page title tags. The SEO process to improve results in this area is know as on-page optimisation.
2. Links into the page (inbound or backlinks)
Google counts each link to a page from another page or another site as a vote for this page. So pages and sites with more external links from other sites will be ranked more highly. The quality of the links is also important, so if links are from a site with good reputation and relevant context for the keyphrase, then this is more valuable. Internal links are also assessed in a similar way. The processes to improve this aspect of SEO are external link building and internal link architecture.
With growing importance of sharing links through social media the search engines now use the number of social mentions to a page and across a site to determine ranking positions. The implications of this are that if companies can get influencers with a larger influence to recommend their content or offers though social networks this can have the dual effect of reaching more people through social graph and improving rankings.
Chaffey, D. and Ellis-Chadwick, F., 2012. Digital marketing: strategy, implementation and practice (Vol. 5). Harlow: Pearson.