Описание тега system-properties
System properties a key value pairs that can be provided to an application, often used to configure it at run time.
In Java there is a set of predefined properties that specify things like operation system, java version and the current user. A complete list can be found at http://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html
One can also provide arbitrary system properties when executing a java application using the -D commandline parameter like the following example:
java -Dkey=value ClassToExecute
Inside the program the properties can be accessed using System.getProperties() or similar methods in the same class.