The reverse side of a coin from heads. With an American quarter, tails is the side with the eagle, or more recently, the side with the state insignia. In the UK tails is the side without the Queen's head on it. On the 2p, 10p, etc., the tails is often the side with bowers/leaves on it.

The tails designation is used almost exclusively in coin tossing.

(thanks for the UK info, nine9)