Crawler based generate results automatically by sending little pieces of software called spiders throughout the internet that look at your site content and links to see if your site has content that matches a search.

Human based are results from a man made database. People submit and review sites and your search looks through that database.

And of course mixed is combination of both getting the results from multiple resources.

There is a more in depth article over at Search engine watch.com. This site is full of helpful resources to help you optimize your web site for search engines.