Societies are regulated by norms and, consequently autonomous agents that want to be part of such societies must be able to reason about norms. However, no reasoning can be doe if agents lack a model of norms that enables them to know how, by complying with norms, some of their goals might be affected. In this paper, a general model of norms is proposed. By contrast with current models of norms, our model emphasizes those aspects that autonomous agents might consider before taking decisions regarding norms. The model is applied to represent the most common kinds of norms that exist in a society, and its effectiveness to design systems of norms is shown.