While these are the two fundamental underlying approaches, many search engines in fact use a hybrid of the two approaches, combined with commercial elements.

Some sites use this information "as is", and periodically update their copies, but many of the bigger search engines, like Altavista and Google, have found ways to blend information from the DMOZ into their own results.

There are also "Pay Per Click" search engines, that are a combination of directories and true search engines. The directories are unusual in that you can pay to be listed for relevant keywords, the amount you pay is based on how much traffic you receive. With Pay Per Click, you only pay for results, as in vistors to your site.

Some search engines have a Pay For Placement option. This means you can pay simply to have your link displayed with the search results. The cost can be based on the number of times your link is displayed, or a fixed fee per day. With Pay For Placement, you pay regardless of whether you get extra visitors, so your risk is higher.

Meta Search Engines, like MetaCrawler, display results which are actually a combination of the results produced by other search engines. Nowadays, these combined results often include results from the Pay Per Click search engines.

Yahoo! is best known for its directory, but has grown to be all encompassing portal with auctions, classifieds, stores, free e-mail services and a customizeable start page. As regards the search at Yahoo!, there are actually two parts:

-- A human edited directory (the web sites section)
-- A crawler built true search engine. Yahoo! currently uses Google for these results.