Great site to experiment with how different tools produce results on the same topic. Index of specialty search categories and keyword search by category. Great resource when you're not sure which search engine or site to use for for your specialized search. Yahoo's meta-index of links to all known search engines sites. The most popular search engines are listed at the top of the page.
Search engines are software tools that use keywords.
There are a variety of them. For the casual user, the differences are subtle. Some deliver results faster than others. Some allow you to search both the Web and Newsgroups. Still others allow you to use multiple search engines simultaneously.