Search engines are practical application of information retrieval system to large scale text collections.
Search engines can be found in many different applications such as desktop search or enterprise search.
Search engines come in a number of configurations that reflect the applications they are designed for.
Web Search engines such as Google and Yahoo! must be able to capture large size of data, and then provide subsecond response times to millions of queries submitted everyday from everywhere. Enterprise search engines for example autonomy must be able to process the large variety of information sources in a company and use company specific knowledge as part of search and related task, such as data mining, desktop search engines such as the Microsoft Vista search feature must be able to rapidly incorporate new documents, web pages and email as the person creates or looks at them as provide an intuitive interface for searching the information