Poseidon is the Greek god of the seas. He is one of Zeus‘ brothers. When the gods had slain their father, Cronus, Zeus and his two brothers divided the universe in three realms. Poseidon was the one that got to rule over the oceans (which was not as a cool as ruling over the sky as Zeus did, but it was better than having to rule over the underworld like Hades). He is often seen as a very powerful deity, only second in power to Zeus.
His power and lineage are not the only traits he shares with his brother, as he was also prone to father many children on many consorts. His children vary from monstrous beings (such as the cyclops Polyphemos) to heroes (such as Theseus) and merman (such as Triton).
Aside from being the god of the seas, Poseidon is also the god of earthquakes, storms, and horses. We think these domains fit him well, as he could have a bit of a difficult character. There are multiple stories in which he quarreled with gods and mortals alike.
His followers would often worship him by building statues of horses. This is also what inspired the Greek armies to build the giant wooden horse of Troy, which would ultimately lead to their victory over the city. A city hated by Poseidon by the way. Coincidence? We think not.