Most coins have a side where the imprint of a person, such as a monarch is impressed - this side is called the "Head" side (since the embossing is of the head of a person). The other side may have any imprint, or none, and is called the "Tail" side.
Generally, one person throws the coin up in the air, and the second person must predict which side of the coin will lay face up after it rests back on the ground. A correct prediction results in a win.
Another variation has the person catch the coin in one hand and slap it on the back of their other hand. Traditionally, the second person calls out "heads" or "tails" while the coin is in the air.
The Australian game of Two-up is closely related, and involves traditionally two half-pennies.
"Cross and Pile" was played in England for many centuries. The cross being the major design element on one side of many coins, and the Pile being the bottom part of the die that was typically hammered into a log and used in the minting process that was accomplished with a small sledge hammer and a die held in the other hand. See also hammered coinage.
A related game, "Cross and Pile" is derived from the Greek pastime called Ostra Kinda, played by the boys of ancient Greece. Having procured a shell, they smeared it over with pitch on one side and left the other side white. A boy tossed up this shell, and his antagonist called white or black (In the Greek, nux kai hmera, that is, 'night and day') as he thought proper, and his success was determined by the white or black part of the shell being uppermost.
See also: gambling