Off-Site Search Engine Optimisation is the process of increasing traffic to your website by supporting and encouraging content creation around the internet and boosting the number of key phrase rich URLs linking back to your website.
Google and the other search engines rate a website based on content (on-site search engine optimisation) and off-site activity. Industry figures suggest that the importance of on-site vs. off-site stands at around 40% on-site activity and 60% off-site. So, if you're trying to optimise your website for a specific keyword you may need both on and off-site SEO to achieve the results you want through organic listings.
The golden goose in SEO terms is a link from a website with high PageRank. Imagine if you got an article written about your website in thetimes.co.uk with its Google PageRank of 7/10. They write an article about your topic of interest and provide a link to your website. Google recognises thetimes.co.uk as a website with great content which changes on a daily basis. By giving you a link, they are recommending their readers visit your site and this therefore increases the importance Google places on your site.
If possible, the link itself should be keyword rich. This means that if you're trying to get to the top of the Google charts with a keyphrase like 'lawyers London', the link back to you should ideally contain the text 'lawyers London'. This is difficult to manipulate unless you employ the services of an article writer (see below).
Article writing is a way of creating content on article websites such as Ezine and Buzzle on topics related to the website you're trying to promote. By writing articles and providing keyword rich back-links, copywriters can build up a buzz around the internet around a topic. Over time, this can help to push the pageRank and importance of the site you're trying to promote.