M.B.
She's still trying to figure out how to express herself and her emotions. My son went through this and now my daughter has just started doing it too.
They don't know as much as we would like them to (because it would be so much easier to just be able to reason with them) so you need to help her figure out her emotions and help her learn how to deal with them in a positive way.
What are her triggers? Is it a specific child? Time of day (tired, hungry) try to figure out what is leading up to her hitting and then remember to talk to her on her level about it.
Don't make her apologize because she has no idea what that means yet. She needs to be empathetic to understand it so help her develop her empathy. When she hits, have the person she hit tell her that it hurts them (or boo-boo or whatever word she has learned to associate with pain), show her with expressions (sad face, crying).
This is a perfectly normal phase that your daughter is going through so I wouldn't be too concerned that your daughter is going to be "mean" or anything like that.
This is a great resource for you, I hope it helps.
http://www.askdrsears.com/html/6/t063900.asp
It goes in depth about troublesome toddler behaviors.