"Multi-thread" search engines: What are they?

Sometimes know as "Metadata" search engines, this type of system will often have a number of additional features such as:

They will also often be backed up by an editorial team of human information specialists, instead of simply relying on automated crawlers.  The number of references displayed will often, however, be limited - normally to only the first ten hits.  This type of tool is often an excellent starting point if you are unsure about which type of standard engine to use, or if you are unsure how to phrase an enquiry.

A recent innovation is offering online access to humans who are expert at searching the web.  You tell them what you want interactively and they find things for you, for example http://www.webhelp.com.