To give a layman's definition:
"You cannot measure/observe something without changing that which you are measuring/observing."
Romantics and artists find this appealing because it appears to refute scientific method. This is far from the truth. It simply means you must account for it.
See also:
Source:
Hawking, Stephen, "A Brief History of Time", Bantam Books, NY, 1988
Last Updated 12.30.03