The scientific community, public organizations and administrations have generated a large amount of data concerning the environment. There is a need to allow sharing and exchange of this type of information by various kinds of users including scientists, decision-makers and public authorities. Metadata arises as the solution to support these requirements. We present a formal framework for classification of metadata that will give a uniform definition of what metadata is, how it can be used and where it must be used. This framework also provides a procedure for classifying elements of existing metadata standards.