As a school counsellor, I often teach a classroom lesson on the differences between kidding, teasing, and bullying. The term "bullying" is used a lot right now, especially with high-profile cases in the media. Sometimes, though, "bullying" is used to describe behaviours that are rude or mean, but not necessarily targeted and persistent. It's important that parents and educators help children understand the distinction between these behaviours so that we can take the right approach to resolving problems.
For some great resources on this topic, check out this post from A Mighty Girl: