How does a pet familiar work?

It's okay, I managed to turn that dog into a familiar after all, even though it took a few tries. I guess it's just a normal spell with a chance of failure, like many of the others, and I've just been mostly unlucky.

Those familiars are really helpful, though, even if they don't do much! My witch is nearly a virtuoso now, and she would have been dead by magic overload three times already if not for her familiar.