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Quantum entanglement is when a pair of quanta behave as one quantum despite how far apart they might be. For example, when energized Calcium atoms de-energize, they produce an entangled pair of photons. Each has a spin, it could be CCW or CW, we won't know until one of them is measured by a detector. But if one is measured to have a CCW spin, we know the other will also have a CCW spin.
Contrary to what the other answer gave, information is not shared as the measuring of the one causes the other to be the same spin direction instantaneously. Which means faster than light and information cannot travel faster than light. In fact, if you think about, what information is there in the fact that the other photon is always instantaneously the same spin direction as the measured one. It is as though the pair were one photon rather than two.
And that's the weirdness of quantum entanglement. It shows that unlike Al's theories of relativity that require locality to influence things, quantum entanglement does not. The other photon is instantaneously influenced when the first photon is measured... at a distance. It's as though there were two boxes and by finding the cat dead in one would ensure the cat was dead in the other, no matter how far away that second box might be... on the other side of the universe is possible.
There are some theories (WAGs mostly) on how this all works. One of them is that quanta work in the probability world of uncertainty. But when we measure some characteristic, like spin, we force their probability density functions to 1.00 for that characteristic.
And, get this, that changes the probability density function for the other quanta; so that measured characteristic is assured for the other unmeasured quanta as well. In other words (why this is a WAG), the probability density function is what travels faster than light. QM is nothing if not weird.
Quantum superposition simply means all possible outcomes superimposed on each other for the probability density function of each quantum. Each possible outcome can be represented as a term in an infinite series of probability terms. As we move farther out in the series the probability of each possible outcome becomes more and more unlikely... converging on zero probability, but never reaching that value. As the series represents all possible outcomes, its sum is exactly 1.000 because it is absolutely certain that one of the infinite number of possible outcomes will happen.
Again the weirdness of QM. One of the possible outcomes for an electron around the nucleus of an atom is that the electron will pop into space time on the edge of our known universe. That probability term is waaaaaay out there in the infinite series; so its very very small. Some would even say it's too small to really matter, so they treat it as zero. Another is that the electron might be located briefly in the center of the nucleus. They are both possible according to QM, but very very unlikely. The more likely place for that electron, where the probability is greatest, is the same distance (or nearly so) that the old Bohr orbital model would have indicated
