Generally, one side has the imprint or embossing of the official head of state or an insignia or emblem and hence it is called the head side. It is the main side of a coin carrying the portrait head of the ruler or a Symbol and name of the country. Since the opposite of the head is the tail, the reverse side is naturally called 'tails'. It depicts the denomination or issue price of the coin with year and mint mark underneath.
Technically, the heads and tails sides are known as the obverse and reverse, respectively. The tail side name possibly originated from the British ten pence coin, the reverse of which shows a heraldic lion with its tail raised.