Muddiest Point from 12/06/10

I'm not really sure about the section with Checksum and Hash Codes.  I understand that, per my notes, a Hash Code is "a unique code based on content of material and is generated based on some mathematical formula."  I guess I just need a little more explanation on these two, since I'm looking over my notes from class and I'm stumped as to how to explain these two concepts to people.