A physically unclonable function, or PUF, is a function instantiated as a physical structure on a chip. The definition is that its actions are easy to evaluate, but outcomes are difficult to predict. The way it works in chips is quite intriguing. It relies upon the minute variances created by the chip manufacturing process at the die level, which means that each chip can respond, in a random and unique way to the challenge, in the challenge-response scenario.
Chip anomalies occur in layers, blocks, devices and the fabric, and can be related to voltage, timing, resistance, etc. Essentially all of these minute anomalies can be analyzed to create a digital fingerprint of the device. Since no two devices will be identical, that makes each device unique with an identifier that is exclusive to the device. And, the nice thing about PUFs is that just about any chip, that has variables which fluctuate, can be used with PUFs.
The most elegant thing about PUFs is that they are inherently unclonable due to their uniqueness. Even if, for some reason, an identical run on identical lines, with the same design rules could be programmed. The intrinsic differences in materials, minute tolerance variants in processes and the electrical characteristics of the semiconductors and other components still would not produce an exact clone.