A search engine is a searchable online database of internet resources. It has several components: search engine software, spider software, an index (database), and a relevancy algorithm (rules for ranking). The search engine software consists of a server or a collection of servers dedicated to indexing Internet Web pages, storing the results and returning lists of pages to match user queries. The spidering software constantly crawls the Web collecting Web page data for the index. The index is a database for storing the data. The relevancy algorithm determines how to rank queries. Examples of major search engines are Google, AOL, MSN and Lycos, etc..