Need a few more details. Was the car at full operation temp? If not, what BlackGT89 said is true. Condensation builds up in the crankcase, especially in cold climates, and if the car is only run for a short period of time, the condensation doesn't have time to evaporate and mixes with the oil.
If the car was at full operating temp, you most likely have antifreeze in the oil. Head gasket or intake gasket leaks are the two most likely possibilities. Have you done any recent work? Heads? Intake?
If it is antifreeze leaking into the oil system, DON'T DRIVE IT. Very bad for your bearings.
Good Luck.