Search Engines display results obtained from a variety of sources. The source may be a third party search provider or it could be through the search engine's crawler.
The chart below shows the relationship between various search engines.
The source may be classified into four types:
Search Engines rely mainly on the primary provider for their search results. Some search engines like google do not depend on third party providers. They display results based on their own algorithm.
Secondary provider: If the primary provider fails to provide the required results, then the search engines depend on secondary providers for their results. It is a backup option.
Directory results: Search engines also display results from directories. An open directory is a volunteer-built guide to the web. It is provided as an option at many major search engines.
These are also called as sponsered listings. In this type of listing, the webpage owner pays a fee to the search engine to list their page.