Search Engine Optimization
SEO is the process of making a website easier to understand by search engines and friendlier for users. The overall goal of SEO is to increase organic traffic from search engines by improving the positions the website appears in the SERPS for various search terms.
At the beginning (think early 2000), SEO was simple but over the years the whole SEO process has become complex.
When optimizing a website for search engines, you have to consider hundreds of rules to satisfy the various search engine ranking factors and at the same time keep your users happy.
To make it easier to handle, the SEO industry has come with different types of SEO. Each type is responsible for a number of SEO rules.