An identity, within the context of a system or application, is a reference to a person or a person-like entity. Identities consist of:
For example, the unique identifier may be a login ID and attributes may include first name, last name, department code, location code and e-mail address. In this case, the login ID must be unique within the system in question. Alternately, a fully qualified e-mail address may be used as a unique identifier. Valid e-mail addresses are unique across the Internet.
The white paper Best Practices for Managing User Identifiers is an excellent introduction to how identifiers can and should be assigned to people in the context of a system or application.