The link provided by Stéphane Rollandin showed me that this question needed to be changed. Someone asked what photons really are, and got many responses that described how the concept varies.
The photon the experimenter in visible-UV spectroscopy usually talks about is an object that has definite frequency ν and definite energy hν; its size and position are unknown, perhaps undefined; yet it can be absorbed and emitted by a molecule.
The photon the experimenter in quantum optics (detection correlation studies) usually talks about is a purposely mysterious "quantum object" that is more complicated: it has no definite frequency, has somewhat defined position and size, but can span whole experimental apparatus and only looks like a localized particle when it gets detected in a light detector.
The photon the high energy experimenter talks about is a small particle that is not possible to see in photos of the particle tracks and their scattering events, but makes it easy to explain the curvature of tracks of matter particles with common point of origin within the framework of energy and momentum conservation (e. g. appearance of pair of oppositely charged particles, or the Compton scattering). This photon has usually definite momentum and energy (hence also definite frequency), and fairly definite position, since it participates in fairly localized scattering events.
Theorists use the word photon with several meanings as well. The common denominator is the mathematics used to describe electromagnetic field and its interaction with matter. Certain special quantum states of EM field - so-called Fock states - behave mathematically in a way that allows one to use the language of "photons as countable things with definite energy". More precisely, there are states of the EM field that can be specified by stating an infinite set of non-negative whole numbers. When one of these numbers change by one, this is described by a figure of speech as "creation of photon" or "destruction of photon". This way of describing state allows one to easily calculate the total energy of the system and its frequency distribution. However, this kind of photon cannot be localized except to the whole system.
The common thread I see is that photons are detected by their interaction with matter, usually molecules. You can tell a photon has been emitted when energy and angular momentum disappear from matter and the matter changes state. You can tell a photon has been absorbed when energy and angular momentum are added to matter and the matter changes state.
It takes a quantum amount of energy to change an atom from one state to another. Theory says that when an electron changes from a higher-energy orbital to a lower one, it releases the same amount of energy and angular momentum each time. And it requires the same amount of energy to change the electron back to the same higher-energy orbital.
The way we measure radiation is quantized. We can only measure it by absorption to quantized matter. There is an assumption that the radiation itself is inherently quantized. Radiation is in the form of quantized photons, which each individually persist until they are absorbed by matter. A photon might never be absorbed by matter but might continue indefinitely, and then it will maintain its individual state indefinitely.
The double-slit experiment shows that an individual photon must be in multiple locations at once. A single photon can change things at multiple locations at once. experiment Given that, why should photons be relevant to Bell's Theorem? A photon is simply not local, except when it is absorbed by matter in exactly one place.
What good does the concept "photon" do us?
How does it help us to think that radiation is in the form of photons?
Atoms absorb or release quanta of energy when they change state. There is no particular reason to think that the quantum of energy that one atom absorbs is the same quantum of energy that another atom released.
If a thousand atoms released quanta of energy at times that result in them all arriving at a particular atom at the same time, there is no particular reason to think that the atom which absorbs a quantum of energy happened to absorb all of the energy that one of the other atoms released, and the quanta from all the other atoms continue their paths unchanged.
The concept of "the photon" causes a whole lot of confusion. What good is it? Does it give us anything useful to make up for that confusion?