This document discusses search engines and discovery tools. It defines search engines as websites that collect and organize internet content to help users locate information, while discovery tools use algorithms to group search results by location, date, domain and other attributes to continuously refine queries. The document explains that discovery tools rank results by similarity rather than popularity to help users find related topics when they don't know exact terms. It provides examples of useful discovery tools like Google's Related operator and sites like SimilarPages and SimilarSites that find similar websites.