Directories rely on the judgement and expertise of the people compiling them. Often, directories have stringent guidelines that sites must meet before being added. Therefore they have smaller, but cleaner, indexes. Hybrid search engines , the newest generation, combine a directory and a search engine. See the Search Engine Alliances Chart for an inside look into parnerships formed by directories and search engines.
Most engines integrate search results and weed out duplicates.
Most meta-search engines default to major search engines. If you have a reference question, try our Electronic Reference Service .