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.

WWW: http://pypi.python.org/pypi/zope.schema