By default, methods that are not overriden are inherited from Object.
If you look at that method's documentation, the return values are "[...] distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer [...])". The method in java.lang.Object is declared as native, which means the implementation is provided by the JVM and may vary depending on your runtime environment.
A small example:
Object o1 = new Object();
Object o2 = new Object();
System.out.println(o1.hashCode());
System.out.println(o2.hashCode());
prints (using my jdk6):
1660187542
516992923
A Hex representation of the hashCode() value is used in the default implementation of toString() by the way: Running System.out.println(o1) prints something like
java.lang.Object@7a5e1077
