Search engines provide access to sites selected by the search engines' software and algorithms. Search engines do not index everything on the Internet, and no two will index exactly the same sites.
All search engines have different search features and behave differently. For this reason it is a good idea to learn to use at least two search engines effectively.
Subject directories provide a subject organization of selected websites.
Meta search engines search other search tools (search engines, subject directories and more) via one interface. Generally the limiting options of the original search tool are lost. For this reason, meta search engines are good for a quick search, or at the beginning of any research project.