They consist of keywords gathered from Web documents and catalogued on a computer. Some engines have accumulated 40 million pages of references, and there are several dozens of these engines. Many of them work tirelessly, snooping through all available sites on the net. They are known as spiders, or crawlers. How the search algorithm has been defined determines how much useful data they bring back.
Most computer users have their own preferences.
I am very fickle and change engine allegiance easily. It is not really a search engine, but a very clever parasite which translates your request to a language easily recognized by all major engines. It then lets them process your request for a short time, and returns information from each. This also gives you the option to go to those sites and search in greater depth. All search engines have a help file to guide you in the proper way to structure queries.