This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The link above to Sun's Java Doc explains a property object as "configuration tool while designing applications". That is to quote "an application capable of downloading files might use a property named "download.lastDirectory" to keep track of the directory used for the last download." Here the map "download.lastDirectory" serves as a configuration tool for some application.