A search engine is a software program that is designed to provide web searchers with relevant information from the Internet based on their search query.
In the simplest terms, crawler or spider-based search engines perform the same tasks. They find, index and provide search results driven by relevancy algorithms. Each search engines’ formula however is unique. That is why performing the exact same search using different search engines often produces different results.
Currently, the most popular web search engines include Google, Yahoo! and MSN / Live search.