You type in a word or words and the search engine scours cyberspace for it and returns a list of links. Medline is NOT an Internet search engine . Click here if you want a Medline search. Search engines differ with regard both to the number of items in their data bases and whether items are annotated to give searchers an idea of the content of a site.
Directories are efficient but limited.
Indexes are inefficient but comprehensive. A thorough search will probably require using two or more search engines. Smaller dot-pitch is better. Even more important is the alignment of the electron guns that create the image. You have to depend on your eyes on independent test.