I don't think you should spank your children as a punishment. As people have previously mentioned, a lot of parents don't explain why what the child did was wrong and just hit them, which only serves to confuse them more.
Obviously I can't comment on the relationships other people have had with their parents and they way they were punished, but whenever my mum smacked me (and it was always my mum, never my dad) I never understood why I was being hit. She also never used to hit me on my bum as is convention (I think) it was usually on my arm or my thigh.
I remember once when I was younger there was a billboard proclaiming something like 'smacking your children is illegal' which I told my mum, I got smacked for telling her how to be a parent. Looking back now, I can see that I was an utter terror to my mother, but she didn't deal with it in the right way, I was too young to understand why I was being physically punished when I'd only used words.