Search engine optimization (often referred to as SEO) is the other half of the search engine marketing (SEM) equation. Unlike AdWords, where you pay for every visitor you get, with SEO you get free traffic. Search engine optimization is the process of optimizing your website to make them accessible to search engines.
SEO has only been around since the mid-1990s, and until recently there were two schools of thought: (1) the first group of SEOs believed that you should “write for machines” and (2) the second group of SEOs thought you should “write for humans.” There are good reasons for both schools of thought, and most SEOs believe that both approaches should be integrated in your SEO campaigns. Basically, the “write for machines” advocates think that a website should be optimized from a technical perspective so that machine algorithms have the best chance of ranking them highly. The “write for humans” advocates say that you should focus only on what your customers think, and you should create content that they enjoy reading and willingly share. These approaches are not mutually exclusive, but as search engines get more and more sophisticated, we expect that the “write for machines” crowd will get less and less relevant, due to the fact that search engines are looking for “human signals” to identify high quality websites.
At a tactical level, SEO consists of both on-site optimization and off-site optimization.