A search engine is a tool to help find specific content. On the web search engines provide results to user queries. Most search engines are created using spiders programs which automatically trawl the internet.
Search engines cannot confirm if the resulting information is correct or accurate. Relevancy filtering is becoming wide-spread. Unfortunately search algorithms are usually proprietary secrets so cannot be examined to determine which of the possible results are found or remain unknown.
Estimates suggest Google is the most popular search engine, followed by Yahoo. Beside these commercial search engines, there exist open-source and distributed search engines, like Grub (distributed crawling with central server), Nutch (centralized crawler) and Yacy (de-centralized, distributed crawler and proxy).