Throwing Around the Term: Cyberbullying

What exactly is cyberbullying? I've been called one and although I reject that label for myself I can certainly see why it's such a issue.

Many people on YouTube and other electronic sites use this term but what defines it? It's usually in the ToS of sites like YouTube but what does it REALLY mean?

Are you a cyber/bully if you respond to negative things being thrown your way?

Have you been called a cyberbully? Have you ever been cyberbullied or just bullied?

What were those experiences like?