Search engines work by using index software and spiders to gather information from web pages across the internet. The index software stores this information in an index database, organizing it to be searched. When a user enters a query, the search engine checks its index and returns the most relevant matching pages. One of the largest search engines is Google, which indexes over 8 billion web pages. Spiders are programs that browse the web to gather data from pages and build the search engine's database.