Tiamat is a gigantic dragon, representing Chaos and the saltwater ocean.
According to an epic Babylonian poem the decendants of Tiamat and Apsu became irritating, so Tiamat proposed that they kill their own offspring. Ea (a water deity) discovered these plans and killed Apsu while he was sleeping.
Tiamat then created an army of monsters to avenge the death of Apsu. Marduk led the gods against them and defeated Tiamat, cutting her body in half. From the upper half, Marduk created the sky, and from the lower half, the earth.