Poseidon: god of the sea
Who is Poseidon?
In Greek mythology, Poseidon is one of the 12 Olympian gods; that is, one of the dozen major deities of the pantheon. He is primarily known for being "god of the sea," but he is also responsible for earthquakes and sometimes called "earth-shaker." Poseidon was established as an Olympian god in Bronze Age Greece (i.e. the second millennium B.C.) and is the brother of Zeus and Hades.
Who is Neptune and what does he have to do with Poseidon?
Neptune is the Roman god of the sea, and the Roman counterpart to Poseidon. (Note: most Greek gods have Roman counterparts because the Roman Empire adopted many of the spiritual beliefs of ancient Greeks who preceded them.) Neptune is the brother of Jupiter (i.e. the counterpart is Zeus) and Plato (i.e. the counterpart to Hades)..
What does Poseidon look like?
Poseidon is often imagined as a man, beyond middle age, but not elderly. He often has curly hair, which is sometimes long, a large beard, and is depicted as muscular. He is sometimes holding a trident (i.e. a three-pronged spear), which he uses in certain mythological stories (more below). Poseidon in art is sometimes shown in, or around, a body of water.
Poseidon: women and children, and Greek cities
What is the story of his birth?
In Greek mythology, Poseidon was born to his father, Cronos, and his mother, Rhea. According to most versions of the story, Rhea saved her son from being eaten by his father when he was born. Poseidon's mother hid him in a flock of lambs to protect him. Rhea then gave Cronos a colt to eat and tricked him into thinking that the animal was what she had birthed. Cronos consumed the colt and Poseidon was saved.
What is known about Poseidon's relationships with women?
Poseidon fathered many children as a result of his many relationships with females. Among his more well-known relationships were with goddesses like Amphitrite, Tyro, Alope, Amymone, and the famed Medusa. Poseidon has relationships with dozens of goddesses, going by the content of myth stories. In another story, Poseidon falls in love with a human woman names Cleito, and her firstborn to him was Atlas, the ruler of Atlantis. And of a crude nature, another story says that after Poseidon raped Caeneus, he fulfilled her request and changed her into a male warrior. Poseidon also had sexual encounters with other male gods.
What was the nature of Poseidon's relationship with Demeter?
When Poseidon chased after Demeter, a goddess of corn and agriculture who is also among the 12 Olympian gods, she turned herself into a horse to hide from him. After he captured her, Demeter gave birth to his offspring, named Arion, who was in fact a horse, but capable of human speech.
What did Poseidon have to do with Athens?
In one story, Poseidon competed with the Greek goddess Athena to become the patron of the city, and the winner would give their name to it. Both of them gave the city a gift. Athena gave it an olive tree, which represented peace and prosperity for its residents. Poseidon gave the city salt water by striking the ground with his trident, which symbolized naval power. The residents of the city accepted Athena's gift and not Poseidon's and she became its patron goddess. The city was then called "Athens" in her honor.
What did Poseidon have to do with Troy?
Hera was the sister of Poseidon (and Zeus). In Greek mythology, she is known for her jealousy of Zeus. When Poseidon aligned with her in a plot against him, Zeus punished Poseidon by temporarily emptying him of his powers and sending him to the city of Troy to serve King Laomedon. The king had Poseidon build enormous walls around the city, large enough to protect it from enemy combatants. Poseidon was supposed to be compensated for his building efforts, but when the king refused to follow through, the sea god sent a sea monster to attack the city. The sea creature was eventually killed by Hercules.
Did people worship Poseidon?
Yes, in certain places. Poseidon had a prominent place in several Greek cities, including Athens where he was second in prestige only to Athena. In the coastal city of Corinth, in the northern Mediterranean, Poseidon was the god of the people. Moreover, sailors often prayed to Poseidon for protection and safety. Even the famous Greek military commander and world-conqueror, Alexander the Great, according to one account, sacrificed a chariot and its four horses by ordering them tossed into the sea in an effort to curry the favor of Poseidon. 
What do Homer's The Iliad and The Odyssey tell us about Poseidon?
According to one sentence in The Iliad, when the world was divided by lot in three, Zeus received the sky, Hades the underworld and Poseidon the sea. In The Odyssey, Poseidon has a home in Aegae. 
Greco-Roman religion beliefs
Greco-Roman religion timeline
1. The Iliad and The Odyssey by Homer
2. Papyrus Oxyrrhincus FGH 148, 44, col. 2; quoted by Robin Lane Fox, Alexander the Great (1973) 1986:168 and note. Alexander also invoked other sea deities: Thetis, mother of his hero Achilles, Nereus and the Nereids