on shell, off shell - what does it mean

From: Ted Sung (teds_at_intex.com)
Date: 06/27/04 

Date: Sun, 27 Jun 2004 22:58:49 +0000 (UTC)

Hi,

I see the terms "on shell" and "off shell" used from time to time.
What do they mean and how are they used?

Thanks,

Ted