I personally don't spank my children, however some parents do and as long as it is not excessive or done out of pure anger or for no reason, then it is not child abuse.
In my opinion, there are more effective forms of discipline, but for some children, a spank is what it takes. Some children don't respond to time-outs, groundings, things taken away. They just don't. And a swat on the bottom, as long as it is not done with force, may be the only way some children respond.
I understand that telling your child that hitting is wrong and then to spank them is kind of contradictory, and that is one of the reasons I choose not to spank. But some parents who spank will tell you there is a difference between a disciplinary swat on the bottom and maliciously hitting someone else out of anger. That is why many parents who do spank warn the child what will happen to them if they continue [insert negative behavior here]and if the child does continue with that behavior, the parent will spank and then sit down with the child and explain why they were spanked and reassure the child that they are still loved.
I was spanked as a child and never viewed it as abuse, I knew what would happen if I continued the behavior, and I didn't listen. The spankings I received were not painful, and I never was marked or bruised or cried.