Search engines such as Yahoo and Alta Vista serve as "yellow pages" for all websites that are registered with them.

They send out "web crawlers" (computer programmes) with regular intervals to visit the registered websites where they catalogue and index the words they find and send this result back to their search engine.

This is effectively what enables you to search for example for "financial+advice+UK" and get a listing of financial advisers in the UK.

It does however only work for websites that are registered and any non-registered websites will not be found unless you happen to know the exact domain name the site is registered under.