These are usually categorized as metasearch tools. They perform the first level search on several of the prominent search sites, collate the results, remove duplications and purify the query to improve their prediction of probable applicability to the searchers needs. A percentage indicating a confidence level is added to the summary of each reported qualifying site. Understanding how a search engine operates is essential to picking the right search tool to meet your specific needs. The same knowledge will help you find what you are looking for while separating the chaff from the wheat.
Some search engines sell placement in their catalogs to advertisers to generate operating revenue.
This can invalidate search results. Each search engine will have a set of characteristics to separate itself from the pack. If you wish to use these tools to count the number of references to a particular topic, you will find wide variation in search engine results. Some engines do not post total results. Some post results found within a given time limit.