Schemas extend the notion of interfaces to detailed descriptions of Attributes (but not methods). Every schema is an interface and specifies the public fields of an object. A field roughly corresponds to an attribute of a Python object. But a Field provides space for at least a title and a description. It can also constrain its value and provide a validation method. Besides you can optionally specify characteristics such as its value being read-only or not required.

See http://docs.zope.org/zope.schema/ for more information.