Search engines are best used to locate a specific piece of information, such as a known document, an image, or a computer program, rather than a general subject . Search engines allow the user to enter keywords that are run against a database (most often created automatically, by "spiders" or "robots"). Based on a combination of criteria (established by the user and/or the search engine), the search engine retrieves WWW documents that match the keywords entered by the searcher. While all search engines are intended to perform the same task, each goes about this task in a different way, which leads to sometimes amazingly different results. Factors that influence results include the size of the database, the frequency of updating, and the search capabilities. Search engines also differ in their search speed, the design of the search interface, the way in which they display results, and the amount of help they offer.