Search Engines are automated computer applications which "crawl" the World Wide Web, indexing all the resources they encounter. Metasearchers, by contrast, search the indices of other search engines. Neither type of resource is universally reliable, because humans don't oversee the resource indices. It's very important to learn to evaluate web resources before using them in an academic project; not all resources are reasonable for all purposes!
Directories, unlike search engines, are not automated; instead, they are hand-selected indices of web sites. These resources are more reliable than the automated search engines, but may not be as "up-to-date." Still, it is important to evaluate any resource you find via these indices.