WCGs mission is to create the largest public computing grid benefiting humanity. You can choose from various work units to crunch: Fight AIDS, cure Cancer or support the Human Proteome Folding projects amongst many. (Previously supported by IBM).
It happens when 1 or more of the WU results don't agree with the others. An extra WU is sent out to get a quorum that agree. If you are in agreement with the others you get full credit. If your result is out of step you have a failed result and only partial credit.
Thanks for the clarification John, as you can see I am a newby to this "crunch" activity, just got in from work and found two of the offending wu's have now been validated :)