Some allow much more sophisticated searches than others. A meta-engine passes runs a search through multiple search engines and then summarizes the results. Meta-engines allow you to search a larger index. Meta-engines only allow you to use the search feautures that are supported by all of the search engines that they search. Meta-engines work well for simple searches, and are less effective for complex searches.
Directories are collections of websites that have been manually indexed or categorized.
They are arranged hierarchically into topics and subtopics. Directories work well when you are interested in broad, general information that fits clearly into a subject. Directories are not as effective if you do not know what subject your question fits into, or if you are searching for an interdisciplinary topic. Basic Search Strategies Explore different search engines. Choose a few that you like and learn those well.