Search Engine Optimization, or SEO, is the process of making a website more visible by ranking it higher in search engines. SEO is generally targeted towards specific keywords, that people are searching for relevant to the website and its content. Successful SEO will result in organic, unpaid traffic visiting the website.
There are various methods in which websites can be optimized for search engines, these include on-page SEO, backlinking and social bookmarking. On-page optimization is the process of making the content of a website search engine friendly. This is generally done by including certain keywords, which you would like to rank for, within the content and providing highly relevant and useful content to the keywords. Other on-page tactics include linking to other pages within your website or other relevant and useful websites with high PR, such as Wikipedia. There are also certain formatting tweaks that can improve a sites ranking, such as including the keyword in your H1 (heading 1). Other on-page techniques are creating a site map, which is a map of your site which the search engine bots can see, creating a relevant meta description with your target keyword in, a meta description is the short blurb about your site the appears under the link in the search engine results. These are some the main on-page optimization strategies that people use to rank their sites.
Backlinking is a strategy that creates links on other sites that link to your site. There are a myriad of strategies and supposed best practices around and everyone seems to be a guru on this subject. In recent times Google has cracked down on spammy backlinking strategies and penalized a lot of sites for this. When Google penalizes a site, it can destroy a sites traffic and income overnight. Again there a million and one strategies out there and there are many different successful ways to backlink. A few of the main ways to avoid getting penalized are: linking from relevant and high PR sites, using varied anchor text and avoid getting too many links too quickly via automatic software, this can set off red flags for the search engines. Again you will find a ridiculous amount information on the internet about backlinking and especially with this there is more the one way to skin a cat and so there are many different strategies that can be used.
Finally, social bookmarking is another strategy that can be used to increase a sites rank in search enignes. Social bookmarking is similar to backlinking but it is using social media, such as Facebook, Twitter, Stumbelupon etc to create links to your site. The search engines realize the power of social media in sharing and rating the quality of information. Hence, the search engines give a lot of weight to the social media presence of a website, so having a large social media presence can definitely help increase a sites ranking. There are many more strategies and much, much more information out there, but outlined above are some the more popular and effective Search Engine Optimization Strategies.