A search engine is a program that searches for and identifies items in a database that match specified criteria. Most web-based search engines work by sending out a spider to fetch as many documents as possible. Another program, called an indexer, then reads these documents and creates an index based on the words contained in each document. When someone enters a query into the search engine, it uses the index to identify which documents are most likely to be relevant to the query and displays them in order of their relevance.

The first modern search engine was WebCrawler, which was created in 1994. Google was founded in 1998 and quickly became the most popular search engine due to its simple design and effective results. Today, there are many different types of search engines available, each with its own unique features and capabilities.