Outbound Versus Inbound?

Inbound marketing is a hot topic right now, in outbound verse inbound. How do you even define outbound verses in bound? I think we should help some people here, because i i doubt that even you and i define them the same. So would running ads to an account be outbound or inbound?  i think it's part of your outboun strategy. Then, what constitutes inbound? It's just, itsiagood question. What's yours?