Short answers, pulled from the story.
Poseidon was originally a chief deity of the Bronze Age worshipped as a master of the underworld and the earth. His title Enesidaon translates to Earth Shaker, linking him to chthonic forces and the power of earthquakes. He was the lord of the palace and a protector of the king before the Olympian order was fully established.
In the region of Arcadia, Poseidon was worshipped as a horse and mated with Demeter in the form of a mare to produce the horse named Arion. This theriomorphic form connected him to the underworld and the creation of springs, as water spirits often appeared in the shape of a horse or a bull. The connection between horses and water remained strong in Greek folklore where horses were believed to have the ability to create springs.
The Athenians led by their king Cecrops chose Athena because she offered an olive tree that provided wood, oil, and food as a symbol of peace and prosperity. Poseidon struck the ground with his trident to cause a spring to erupt, but the water was salty and of little use for drinking or agriculture. Enraged by this rejection, Poseidon sent a monstrous flood to the Attic plain to punish the Athenians for their choice.
Poseidon is the primary antagonist in the epic poem the Odyssey who punishes the hero Odysseus for blinding his son the Cyclops Polyphemus. He causes a decade of storms that delay Odysseus's return to Ithaca and result in the complete loss of his ship and numerous companions. Despite being a terrifying and avenging figure, he is also the protector of seafarers who pray to him for a safe voyage.
Poseidon was the father of the great hero Theseus who was said to be the son of Poseidon and the Troezenian princess Aethra. He was also the father of the Minotaur a half-bull half-human creature who fed on human flesh and the father of the winged horse Pegasus. Poseidon granted Theseus's wish to destroy his son Hippolytus by sending a terrifying sea monster to spook the man's horses.
The temple of Poseidon at Helike in Achaea was destroyed by a tsunami in 370 BC and sank beneath the sea taking the city with it. This disaster was a stark reminder of Poseidon's power to destroy as well as to create and led to the worship of the god as a bringer of safety. The temple at Isthmia near Corinth was built in the early 7th century BC and was one of the pioneering buildings featuring Doric architecture.