The document discusses various sources of traffic to websites, with a focus on search engines. It defines search engines as internet tools that search large indexes of web pages for user-specified terms and display matching results according to relevance. Examples provided include Google, Yahoo, MSN and others. The document also discusses keywords, how they relate to search results, and search engine optimization techniques including on-page optimization of individual pages and off-page optimization that occurs outside the website, such as link building.