Most of the times kids hit not to hurt someone but at this stage they are trying to discover things. Many a times they may hit because of frustration. Sometimes they may hit to see other person's reaction.

You need to handle your child properly at this stage. My daughter used to do similar things when she was about 2 years old. I used to follow the following rule.
1) Told her that its a bad habit to hit others and it hurts.
2) If she did not stop ask her to stand in a corner for some time, many a times she would calm down.
3) When she was totally out of control just move away from her and let her calm down.

You can try this out with your child.
Hope this works.
All the Best.

i can be wrong. but i think kids learn from what they see around them.
Few things you can do-
1) do not watch tv programs that have such scenes
2) do not use hand or other object to hit your kid. they have tendency to pick up such thngs and use them whenever they get the opportunity tyo do so.
3) watch out for his company. the kids he plays with. the school, in case he goes to school
4) try to divert the attention of your son. do this sincerely for at least 2 months. it will take a while for them to forget such habits. but the good thing is that they do forget things and pick up new things. after all kids learn new things everyday. so they can only practice some of them. and give up on the rest.

in this case my thoughts are same as of carebaby. usually we think that kids are too small to understand anything sensible. this is not correct.
they need to be told for more than one time. but in the end they do understand everything.

please talk to son whenever he does that. tell him in very low pitch voice how this is not correct. use few stories to explain the same thing. like how people who don't hit others wre rewarded.
the key is talk to him about this as much as you can. make him feel more responsible for his actions.

i do not agree. i have completely different opinion.

in case of babies nothing makes more sense than ignoring it completely unless they are going to harm themselves.

just do not pay any attention to whatever he does. by this i do not mean you let him do all that he wants to do. just divert

his attention. he does not want to hurt. all he wants is get your attention. and through experience he has learnt that nothing gets more attention than this.
you have to change this. you have to let him know that this does not get enough attention.

gunimam,
I have a completely different view on your child's 'problem' than the ones suggested. Hitting is a way toddlers use to try to make contact with others before they develop other social skills. It seems to me that your son is becoming interested in other people but he is too young to let them know his interest. Kids of this age just start discovering the world around them and they like to see the reactions of different people. For example, if you cry when you are hit, your kid will hit again to know whether the same reaction is being displayed or a different one. They need to repeat the activities over and over again to know what they can expect. This is one way that kids learn. Please try to understand your child!