Poseidon was the god of the sea, earthquakes and horses. Although he was officially one of the supreme gods of Mount Olympus, he spent most of his time in his watery domain. Poseidon was brother to Zeus and Hades. These three gods divided up creation. Zeus became ruler of the sky, Hades got dominion of the Underworld and Poseidon was given all water, both fresh and salt.
Although there were various rivers personified as gods, these would have been technically under Poseidon's sway. Similarly, Nereus, the Old Man of the Sea, wasn't really considered on a par with Poseidon, who was known to drive his chariot through the waves in unquestioned dominance. Poseidon had married Nereus's daughter, the sea-nymph Amphitrite.
The Romans' name for Poseidon was Neptune.
In dividing heaven, the watery realm and the subterranean land of the dead, the Olympians agreed that the earth itself would be ruled jointly, with Zeus as king. This led to a number of territorial disputes among the gods. Poseidon vied with Athena to be patron deity of Athens. The god demonstrated his power and benevolence by striking the Acropolis with his three-pronged spear, which caused a spring of salt water to emerge. Athena, however, planted an olive tree, which was seen as a more useful favor. Her paramount importance to the Athenians is seen in her magnificent temple, the Parthenon, which still crowns the Acropolis. The people of Athens were careful, all the same, to honor Poseidon as well.