How do search engines differ:
Web Crawler Search Engines: Use spiders or bots to travel to each page and gather key words the on the page and adds it to its database. The database is than searched.
Meta Search Engines: search engines that search search engines for results. These search engines search indices and directories of other search engines for their results.
Directory Search Engines: human created directories of web content
Hybrid Search Engines: a combination of both crawler search and directory search engines.
Question Answering Engines: search by asking a question in natural language
Video: How Search Works
Questions to answer:
Demystifying Google Search: links to 26 short videos on "How to" search using Google