The “deep web” is the part of the internet that is not typically indexed by the major search engines. It includes searchable public record databases; company databases; non standard file formats ( like image files, audio files, adobe acrobat files etc. ). The deep web contains a lot of useful information for skip tracers and private investigators. A good overview of the deep web can be found at University At Albany, New York .
A directory of deep web and specialized web search engines can be found at 411 x 411 Deep Web and Specialty Search Engines .