The difference is 180 degrees of longitude. They're two different [imaginary] lines  on the earth's surface, directly opposite each other. The Prime Meridian connects  the north and south poles and runs through Europe and Africa. The International  Date Line also connects the north and south poles, but runs down the other side  of the earth, through the Pacific Ocean. If you stand at any point on either one of  them (except at the north or south pole), then you can't see any point on the  other one.