One of the three sons of Cronus and Rhea, Poseidon is the god of the sea. He is often seen holding a trident, with which he caused earthquakes. His wife was Amphitrite, a Nereid, who gave birth to Triton.
When the early Athenians were deciding upon a name, two gods vied for the town to be named after them. The gods were Poseidon and Athena. Poseidon gave the citizens the horse, and Athena gave them the olive. The Athenians chose the olive, industry, over the horse, war.
It was Poseidon's sexual conquest of Medusa led to her being turned into a Gorgon. When she was slain by the hero Perseus, the winged horse Pegasus and Chrysaor emerged, presumably the unborn children of Poseidon.
Poseidon is also the father of Theseus, the hero of Athens, as well as causing the creation of the Minotaur, which Theseus killed. Pelias, another of his sons, sent Jason on the quest for the golden fleece.
Poseidon played a major role in the Trojan War. He and Apollo plotted to overthrow Zeus, and then become kings themselves. Zeus found out, and crushed their rebellion before it began. As punishment, Zeus forced Apollo and Poseidon to work for a mortal for wages. The man Zeus chose was Laomedon, the fifth king of Troy. Laomedon gave Apollo and Poseidon the task of building walls around Troy. Laomedon forced the pair to continue building greater and greater walls, giving Troy its legendary walls, which kept out the Greeks for ten years during the Trojan War. Upset by Laomedon cruel treatment of them, Apollo sent a plague and Poseidon sent a sea monster to wreak havoc on Troy.
During the war itself, Poseidon fought on the side of the Greeks, presumably still holding a grudge against them. After the war, however, he repeatedly sought the destruction of Odysseus. This was on account of Odysseus' attack on his son Polyphemus, not a hatred of the Greek cause itself.
The Romans identified Poseidon with Neptune, their own god of the sea.