Policy is the way that you intend to do this. Practice is actually doing it. Well put in plain English, and succinct. Theory, policy and practice or practice, policy and theory from the basic pattern for translating our ideas about taking care of our society. Pattern by which we go from an idea to an action or from an action to an idea.
Agencies are a branch of our government to help put our ideas into practice.
Good inclusion of explanation of government's role. This is an example of Dang Cao and me working together to produce an answer that gave some details that ought to help everyone understand. I framed the answer with his words as much as possible, editing where he needed help to extend the ideas. Let's use for example the theory of communism or socialism as a theory of society. In such a society everyone should be equal.