What makes an iris scan unique?
The iris is the colored ring of muscle that opens and shuts the pupil of the eye like a camera shutter. The colored pattern of our irises is determined genetically when we're in the womb but not fully formed until we're aged about two. It comes from a pigment called melanin—more melanin gives you browner eyes and less produces bluer eyes. Although we talk about people having "blue eyes," "green eyes," "brown eyes," or whatever, in reality the color and pattern of people's eyes is extremely complex and completely unique: the patterns of one person's two eyes are quite different from each other and even genetically identical twins have different iris patterns.
How does iris scanning work in practice?
To get past an iris-scanning system, the unique pattern of your eye has to be recognized so you can be positively identified. That means there have to be two distinct stages involved in iris-scanning: enrollment (the first time you use the system, when it learns to recognize you) and verification/recognition (where you're checked on subsequent occasions).