Short answers, pulled from the story.
The earliest written record of the name Poseidon appears on Linear B clay tablets from Mycenaean Greece, inscribed as Po-se-da-o or Po-se-da-wo-ne. This form corresponds to the later Homeric Greek Posidaōn and indicates a Pre-Greek origin rather than an Indo-European one.
Poseidon shifted from tamer of horses to protector of sailors and fishermen as Achaeans migrated from the interior to coastal Ionia. Natural philosophers like Thales and Anaximenes explained earthquakes as erosion by waters disappearing into the earth, transforming the inland god of waters into the ruler of the ocean.
The Temple of Poseidon at Sounion was built in 425 BC following an earlier version destroyed by Persians in 490 BC. This hexastyle structure had six columns across the front and fourteen along the flanks with an Ionic frieze running around the interior of each end of the pteron.
In the standard version, Poseidon was born to Titans Cronus and Rhea as the fifth child out of six. His father devoured each infant immediately after birth until Rhea deceived him into eating a rock wrapped in a blanket instead of Zeus.
In Athens, he competed against Athena for patronage by striking the ground to create a salty spring. The Athenians chose Athena's olive tree gift over his salt water offering, causing Poseidon to send a monstrous flood to punish them.