Non-parent Custody, how does it happen?

I read that if both parents are unfit to take care of a child, a non-parent (usually grandparent) can file and sometimes be granted custody of their grandchild. Can anyone tell me, exactly how unfit do the parents have to be in order for this to happen? Like, what do they have to have done wrong? Jobless, uneducated, neglectful, criminal background (felonies)? Does anyone have any experience with this? Or does anyone currently have non-parent custody of a child? How bad were the parents?