system
Given the following post, respond as if you're commenting on Reddit:
human
What is spin in Quantum Mechanics? What function does it serve in QM?
gpt
It's just an intrinsic form of angular momentum. 

I know that may not seem like a very satisfying answer, but spin is just some fundamental quality that's tacked onto particles - mass, electric charge, spin - they're all just numbers that describes how the particle will behave (though spin is slightly more complicated than charge or mass). 

It's an incredibly important piece of quantum mechanics - charged particles with spin have an intrinsic magnetic dipole moment, sort of like a tiny bar magnet. Additionally, spin-statistics governs how systems with multiple particles interact. For example, half-integer spin particles can only be in the same state if they have opposite spins, which is the origin of the Pauli Exclusion Principle for fermions. Bosons, on the other hand, don't give a fuck, and you can pile a bunch of them into the same state - this is the principle behind Bose Einstein condensates, and why you don't have to worry about light bumping into other light.