Quantum spin

It is easy to hide ignorance behind mathematics and arcane terminology, and this is a common trick in explanations of quantum mechanics because it is very hard to understand. The fact is, when looking at objects around molecular sizes, the usual rules of interaction no longer apply. A great example is absorption of energy. When you bang a bell with a hammer, no matter how hard or soft you bang it, the energy you use will be absorbed and a sound will be generated. This is not true at the atomic scale. You can shoot billions of photons of energy at an atom and it will have no effect at all unless the amount happens to coincide with one of the energy states of that atom, then 'resonance' happens and a photon is absorbed. After absorption of the energy of the photon the atom transitions to an excited state. What happens while the atom is excited depends on whatever the transition was, usually the atom relaxes using some other mechanism and the energy is released again.

Spin is one of those quantum states that undergo transitions after absorption of the right amount of energy. It is called spin for historical reasons, and the current theories equate it to some kind of angular momentum, but that is just the current theory and it could change if someone figures out what it really is.

Brief overview of spin. Mainly taken from the nice explanation here: Spin on wikipedia

Table of particle spins