At that age children aren't very capable of thinking about how their actions might make others feel before they do it. So unfortunately it's hard to explain to a two year old that when she bites you (or others) that it hurts that person, because she's not really going to stand there next time and think about how it's going to make you feel first. They seem to remember consequences a little better though, so yelping in pain then either putting them down on the floor or moving away from them can help a little. They don't like that and a negative response gets attached to the behaviour.

I'm a firm believer that biting back won't help. They are little copy-cats and like to do what mummy (or anyone else around them) does. Plus, if Mummy is allowed to bite, then why can't they? I kicked myself the other day when my son was screaming and I yelled "No screaming!" at him. Great way to parent by example, lol.