Well with playing cards: Face Up: The front is showing (with the number, A,K,Q,J,10,9,8,etc.) Face Down: The back is showing (with the design) If you are lying "face down", you are on your belly.

Tor's explanation can also be expanded to include most 'things' in English. For example: anything that has a front, like a book, coin, etc., would be considered to have a 'face'. Therefore, just about anything can be face up or down. "The book was lying face down on the table so Joe could not read the title."