Each object class is defined in terms of attributes, or specific pieces of information that describe the object. For example, the NDS schema contains an object class for users, called User. This User object class defines many attributes, such as the user's name, telephone number, address, and group memberships. An object is a specific instance of an object class. You can extend the schema by defining new object classes.
Attributes hold a piece of information that describes an object.
This information is the attribute value. Attribute values can be multi-valued. That is, you can assign more than one value to a single attribute. Administrators and applications define values for the attributes. If an attribute is optional, administrators or applications don't need to assign a value to create an instance of the object.