Fidget Spinners Are Everywhere! What Are They?

If you are a parent, you already know. The struggle is real. Two words. Fidget spinner. Who and why is what I want to know? I do not know if I should shake the hand of it's creator or throw it back in his or her face. lol For real though, every single child I know, including mine has 1 or more of these. 

It is a fad that looks like it's here to stay for quite some time. My question is, what are they for? I've heard a couple of different things, plus are they really allowed in some schools? If any parents who are far more familiar with these little toys/gadgets our children are obsessed with, please feel free to clarify. Do you know the fidget spinner struggle?