Properties are
passe; Preferences are the current rage. That said, properties are still useful for quick-and-easy settings.
Basically, a Properties file contains key/value associations:
ConnectionURL=jdbc
dbc:MyDatabase
DriveClass=sun.jdbc.odbc.JdbcOdbcDriver
Password=pass_word
UserName=web_user
This is an example of a properties file specifying a Database connection.
It is saved in the classpath with a .properties extension. (Say, as "MyDatabase.properties" in the "mypackage" package.)
You use it by using the java.util.(Property)ResourceBundle class
Of course, you can do other things with it as well. See the java.util.PropertyResourceBundle documentation for more information.
Piscis Babelis est parvus, flavus, et hiridicus, et est probabiliter insolitissima raritas in toto mundo.