Is it bad to crack your neck or back?  

Maybe not. Cracking a joint moves the parts of the joint, with the cracking noise coming from cartilage surfaces sliding, from movement of a little air-space from one place in the joint to another, or from some other non-traumatic event. A person with no joint pains before or after cracking a joint is probably not damaging the joint. However, making it a frequent habit has uncertain long-term effects.
No. Just like popping a knuckle joint, you can crack your back or neck as well. This generally shouldnt cause any problems.