The difference between Commonwealth and a State?

What is a commonwealth and how does it differ from a state?

A longer title.

Cecil’s view:
Why is Virginia (and MA and PA and KY) called a commonwealth?

And the comments of the Teeming Millions:
I forgot…How many states are there?
[url=“http://boards.straightdope.com/sdmb/showthread.php?threadid=14640”]Massachusetts’ classification as a… commonwealth?

50 US States or 46?

I forgot…How many states are there?