These sites do not crawl the web themselves to build an index. Instead they allow searches to be sent to several search engines all at once. The results are then unified. They may either present results from each engine one after the other (parallel search), or blend them, removing duplicates.
This category is strictly for metasearch-based sites.
Sites that provide a more comprehensive "start page" should be submitted to the Web Portals category.
Search tools that do not combine search results into a single stream, but rather return them separately from each index or directory should be submitted to the Multi-Search category.