Radiant Aunt: From a site that can say it better then me....

"Schroders cat was the idea that if you put a cat in a box with a radioactive substance and as soon as an electron from the substance hit a plate, it woudl release a gas that would kill the cat. THe theory is, that the cat is simultaneously alive and dead because the experimetor does not know if the cat is dead or alive. Therefore, the experimentor could think the cat is alive when it is really dead, or vice versa. Because of this, the cat is both alive and dead."

IE you don't know.