Policy-based computing is being studied and applied in a number of different domains by industrial organizations and academia. Standards are also being pursued in many standards bodies (e.g., IETF, DMTF, OASIS, GGF and W3C). This has led to the creation of different policy languages and policy implementations. In this paper, we attempt to provide a framework for the classification and comparison of the different approaches that such languages and systems have taken. This is important because it helps in the understanding of the capabilities of policy systems, and in clarifying what they can and cannot do. It also identifies basic issues in translating policies from one language to another, and in transforming policies specified for a particular system implementation into policies that can be enforced by a different system implementation.