Some of the search tools discriminate upper case from lower case, others store all words without reference to capitalisation. The only drawback with keywords searches is that they have a tough time distinguishing between words that are spelled the same way, but mean something different. Concept-based search systems, using a technique called clustering, try to determine what you mean, not just what you say. It sticks to the numerical approach to build clustering systems. Excite's software determines meaning by calculating the frequency with which certain words appear.
All search engines function against a database, but various factors influence the results from each.
Size of the database, frequency of update, search capability and design and speed may lead to amazingly different results. There are also metasearch sites or metacrawlers that send searches to several search engines. Since metaseach engines do not allow for input of many search variables, their best use is to find hits on obscure items. Some of the best known ones are DogPile, Inference Find, Mamma, MetaCrawler and SavvySearch. Almost all search engines, besides using natural language searching, also employ Boolean searching, field searching and other syntax to present the hits.