Who Was Cronos?:
Cronos was the youngest of the first-generation Titans. More significantly, he sired the gods and goddesses of Mt. Olympus. The first-generation Titans were children of Earth and Sky. Earth was known as Gaia and Sky as Ouranos or Uranus. The Titans weren't the only children of Gaia and Ouranos. There were also the 100-handers (Hecatoncheires) and the Cyclopes/Cyclops. Ouranos imprisoned these creatures, who were Cronos' brothers, in the Underworld, specifically in the place of torment known as Tartarus (Tartaros).
Cronos Rises to Power:
Gaia was not happy that so many of her children had been locked up in Tartaros, so she asked the 12 Titans for a volunteer to help her out. Only Cronos was brave enough. Gaia gave him an adamantine sickle with which to castrate his father. Cronos obliged. Once castrated, Ouranos was no longer fit to rule, so the Titans awarded the rule to Cronos, who then freed his sibling Hecatoncheires and Cyclopes. But soon he re-imprisoned them.
Cronos and Rhea:
The Titan brothers and sisters married one another. The two humanoid Titans, Rhea and Cronos, married, producing the gods and goddesses of Mt. Olympus. Cronos was told that he would be deposed by his son, just as he had deposed his father. Cronos, determined to prevent this, went to extreme preventive measures. He devoured the children to whom Rhea gave birth. When Zeus was about to be born, Rhea gave her husband a stone wrapped in swaddling to swallow, which is hard to believe since Cronos was supposed to be a smart Titan.
Rhea, clearly about to give birth, raced to Crete before her husband could tell she had deceived him. Zeus was raised there safely.
As with most myths there are variations. One has Gaia giving Cronos a horse to swallow in place of the sea and horse god Poseidon, so Poseidon, like Zeus, was able to grow up safely.
Exactly how is debated, but somehow, Cronos was induced to take an emetic, after which he vomited out the children he had swallowed.
The regurgitated gods and goddesses got together with those gods like Zeus who hadn't been swallowed, to fight the Titans. The battle between gods and Titans was called the Titanomachy. It lasted a long time, with neither side having an advantage until Zeus re-freed his uncles the Hecatoncheires and Cyclopes from Tartarus. Then, when Zeus and company won, he shackled and imprisoned the Titans in Tartarus.
Zeus released Cronos from Tartarus to make him the ruler of the Underworld area called the Islands of the Blest.
Cronos and the Golden Age:
Before Zeus came to power, man had lived blissfully in the Golden Age under Cronos' rule. There was no pain, death, disease, hunger, or any other evil. Mankind was happy and children were born autochthonously from the earth. When Zeus came to power, he put an end to mankind's happiness.
Despite his being fooled by the stone in swaddling clothes, Cronos is regularly described as wily, like Odysseus. Cronos is associated with agriculture in Greek mythology and honored at a harvest festival. He is described as having a wide beard.
Cronos and Saturn:
The Romans had an agricultural god named Saturn who is in many ways the same as the Greek god Cronos. Saturn is married to Ops who is associated with the Greek goddess (Titan) Rhea. Ops was patroness of wealth. The festival known as the Saturnalia honors Saturn.