Port Locations

Ok, so I float around a few different forums and everyone has something different on the subject.

Could someone please explain port loading?

Why would you want have the sub in different locations in relation to the port? (close, far, centered on the face, etc)

For example lets say for arguments sake; lets just have the port and sub on the same plain on the box for this discussion.

I'm always trying to learn things to help me make louder, better boxes for my customers.

I figure understanding this phenomena will help.

Thanks in advance