Types of Identifiers
The building blocks of identification are "identifiers." Identifiers are facts that distinguish people and entities from one another. What we often call a "characteristic" or an "attribute" becomes an identifier when it is used for sorting and organizing people and institutions in our thoughts and records.
For example, an attribute of a person named Thomas is the fact that his name is "Thomas." Thomas may have other characteristics or attributes, such as his owning a T-shirt, but his name will be more commonly used as an identifier. Indeed, names exist to be identifiers.
A T-shirt is unlikely to be used as an identifier very often, though it can be. In a remote village of sub-Saharan Africa or high in the Himalayas, for example, ownership of a T-shirt may be a distinguishing characteristic and this a useful identifier. People might call Thomas "the guy with the T-shirt." Wearing a bright blue ascot in such a place might be an extremely distinct attribute and thus a worthy identifier. Perhaps there, as in most places, it would earn the effete Thomas a punch in the stomach.
So identifiers are facts used to sort and categorize people and entities from one another. Although there are many different kinds, identifiers have traditionally been grouped into three categories: something you are, something you know, and something you have.1 An additional category -- something you are assigned -- is sufficiently distinct from the others to be treated separately. These categories are not hermetically sealed from one another, but they are a helpful way of organizing the world of identifiers. [...]
"Something-you-are" identifiers are characteristics that are inherent in a person or attached to his or her physical body. They include hair color, fingerprints, DNA, voice, signature, and other biometrics,
"Something-you-are-assigned" identifiers include names, titles, numbers, and addresses. These identifiers are socially defined. That is, they exist because of traditions in human societies that define and organize people, places, statuses, times, and so on. These identifiers are associated with people but not inherent or attached.
"Something you know" is the characteristic of having some distinct knowledge -- usually knowledge that few others have. Common examples of something-you-know identifiers include knowledge of passwords and mothers' maiden names.
Finally, a "something-you-have" identifier is the characteristic of possessing some distinct item. Identification cards are the most common example. Something-you-have identifiers are often called "tokens" because there are many examples beyond cards. In the future, something-you-have identifiers may take many forms and use many different technologies.
Different identifiers have different qualities. Some identifiers are fixed to a person -- or at least they are likely to stay fixed -- for life, such as the fingerprint or mother's maiden name. Others are transient and exist only for a few moments, such as "bending over tying his shoe." Some identifiers are unique to each individual (DNA), whereas others are quite commonplace (brown hair).
-- Jim Harper
from "Identity Crisis: How Identification is Overused and Misunderstood"
Quoted on Tue Oct 23rd, 2012