Schrödinger's cat is a thought experiment devised by Austrian physicist Erwin Schrödinger in 1935. A cat, along with a flask containing poisin, is placed in a sealed box. If an internal Geiger counter detects radiation, the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when we look in the box, we see the cat either alive or dead, not both alive and dead.
For a simpler and more amusing explaination please visit Miss Cellania's page.