All libraries, archives, repositories use some sort of classification system to catalog their collections. One way to catalog is to use the Dewey Decimal system. This system was invented by Melvil Dewey in 1876 and has been modified throughout history including the newest revision in 2004. For more information about how this system works, consult Wikipedia at http://en.wikipedia.org/wiki/Dewey_Decimal_Classification or the kid's site on Dewey at http://library.thinkquest.org/5002/.
Sometimes it is helpful to wander down a stack of books to see what might be of interest to you. To do this, it is helpful to know what type of books you will find in certain numbered sections. A helpful brochure prepared by the Genealogy and Local History Branch of the Mid-Continent Public Library in Missouri can be found at http://www.mcpl.lib.mo.us/genlh/dewey.pdf. This brochure provides the Dewey numbers for everyone state and selected subjects. If you are working on Missouri ancestors, the 2nd page of the brochure provides the call numbers for each county in Missouri.
Another list from the St. Louis County Public Library at http://www.slcl.org/branches/hq/sc/sc-frqdewey.htm.
A listing of topics and their corresponding Dewey call number Library of Congress Classification can be found at http://www.questionpoint.org/crs/html/help/en/ask/ask_map_lcctoddc.html.