Not Poetry, This Time.

In television, what is the difference between a Soap Opera and a Drama?

I've been having a discussion with my flatmates on how to class Norman's favourite addiction, ER. I've always thought of it as a soap, because it's focusing on internal story lines between the characters, but Norman says it isn't, because it's not full of people wailing about having just married their brother by accident. Also, there was some discussion of overarching story lines.

What is the actual distinction? Was Babylon 5 a soap?