When any two quantum objects, such as electrons or photons, come into contact with each other, their quantum states (the mathematical information describing their properties) combine, or become entangled. Thereafter, their fates remain intertwined, however far apart they move in the future. This bit is not so strange, perhaps, since it is easy enough to believe that, having a shared past means two entities will have affected each other's properties in some way at the time of their interaction. The effect of this interaction can still be seen when we check the particles afterward.However, entanglement becomes much stranger than that! In the quantum world, entities can exhibit two or more conflicting characteristics simultaneously, such as spinning in opposite directions at the same time. This is called "superposition." Now, if a photon, say, is entangled with another it can "infect" it with its superposition so that they are both in superpositions. However, once we look at one of them, this constitutes a measurement, and we force the photon to decide which way it is spinning. But, because it is entangled with its distant partner, we also force the other photon to make the same choice. This happens instantaneously, even if the two photons are now millions of miles apart.
by Jim Al-Khalili, 30-Second Theory