Discusses how search engines (1) select content: human/automated, selective/comprehensive, subject/open category, electronic/print, and peer-reviewed/or not; (2) compile information: human/automated, filtered/unfiltered, peer-reviewed/or not, and evaluation/review added/or not; and (3) search: keyword, Boolean/proximity operators, limit to fields/tags, and relevancy ranking. Outlines relationships among different types. (PEN)