Difference between role and policy into a java ee security realm

Basically I have read: http://ift.tt/29nsjqr

http://ift.tt/29gdIJL

But I can't figure out what is the difference between security role and security policy.

A role is a set of permissions in order to access and according what I have read, security policy respond the question "Who has accesss?" but that doesn't mean anything for me. Can anybody explain me, please.