Actually, the workings of search engines such as Google, Teoma, All the Web, and AltaVista (just to name a few) is proprietary. But we can discern from experience some generalities:
With Google, enclose phrases in "quotes". This forces adjacency and limits the results. Ex.: "joint committee on printing".
Often it is helpful to limit your search to a specific file format such as Adobe Acrobat (PDF), Word (DOC), PowerPoint (PPT), Excel (XLS), etc. To do this, type filetype:[extension]. Ex.: digital divide and filetype:ppt.
A powerful way to restrict search results is to limit by Internet domain. For example, you could restrict results to educational domains (.edu), to government domains (.gov), or to Canada (.ca). To do this in Google, add site:edu to your search to limit to US educational domains.
Another useful trick is to search a secondary-level domain. To search for information on Museum Studies on University of Denver sites, search like this in Google: site:du.edu museum studies.
This strategy is very effective when looking for authoritative information from known Web sites such as un.org. To search for information on "child soldiers" from the United Nations site, search Google like this: site:un.org child soldiers.