The "taps" I have given ds are as hmb says, just that - to get his attention. They don't hurt him - or at most a very slight sting (in the same way that you might "smack" a friend's hand, who was trying to steal your last Rolo! ) - but enough for him to know that he has done something wrong - or that what he was about to do was unacceptable or dangerous. It is also NEVER done after the event. That WOULD be abuse - as the whole point is to equate cause and effect.
In my case, the reason why ds has never asked or needed explained about smacking is that since he has begun communicating properly, (he only really started talking about 4 months ago) it has never been necessary. All the alternative methods have been sufficient.
However, if he were ever in the kitcehn with me and either tried to turn on the gas hobs, or tried to reach towards a gas hob that was on, and, say, I had a boling kettle in one hand, I would have no hesitation in smacking away the offending hand with my "free" hand. He would probably cry - but I would then immediately explain that waht he had done was dangerous and why he musn't do it again. I AM teaching him those lessons now - avoidance of such a sitation is obviously the best approach - but I have tried to imagine a scenario in which I might still smack him.
There has been discussion about why it is accpatable for children but not for adults and some people have pointed out that not all things are suitable for all ages. I have on occasion had to walk around shops - or across car parks - holding ds' hand in a vice-like grip to stop him running away. He will be wailing that it hurts him - but will attempt to run away if I loosen it. Such a grip would not be acceptable to an adult. However, an adult has a sense of danger and is also (in most cases!) responsible for their own actions and safety. A child has no such concept - and we, as their parents - have a responsibility to keep them safe from danger.