Why? Cold runs rough

I know we all have experienced it, I know most all cars are this way. What I don't know for sure is why does a car run rough until it is warmed up?

It's possible there is more than one reason, but because of a choke being the answer in most cases I would assume it has to do with air/fuel mixture. But I'm trying to get to the root cause and effect.

Can anyone help explain this?

Thank you
Joe.
Author: Barfly