The first directories and search engines required precision in the order and syntax of keywords in a search query. Every word, with the exception of common stop words such as and, a, and with, was matched to produce the list of results. As a result, users needed to think carefully about which words to use and the best order to enter them.
With natural language searching, user can submit a query in the form of a plain-language question, such as

Where can I find information on...?

What is the...? or 
How large is...? or 
Where is...? 

Results from natural language searching vary widely from one search topic to another, but this method should be considered when keyword searching fails to turn up enough relevant results.
Ask Jeeves was designed from the outset to accept natural language queries.
Recently, AltaVista offered users the option of entering search queries in natural language form as well as the traditional form.