In ancient Greek mythology, the Titans were a primeval race of powerful deities, descendants of Gaia (Earth) and Uranus (Sky). They were important entities that came before the Olympian gods. According to Hesiod's Theogony, there were twelve original Titans:
1. Oceanus: The Titan of the ocean. He was depicted as a huge river encircling the world. The eldest of the Titans, Oceanus was married to his sister Tethys. Together the two produced over 6000 spirits of the oceans and streams, known as the Oceanids. In fact, Oceanus and Tethys were far too fertile, and their union began to cause floods so they divorced to stop all the damage they were causing. He gave over his realm to Poseidon after the rise of the Olympians, but Zeus allowed him to continue to live as a simple god of the ocean.
2. Tethys: Oceanus's wife and the Titaness of the sea. She was the mother of the chief rivers and the Oceanids (water-nymphs). When Cronus became paranoid and his wife, Rhea, wished to protect her children, she brought Hera to her sister Tethys who raised her as her daughter. Later, as a favor to Hera, Tethys punished Callisto and Arcas, a lover and child of Zeus, by forbidding their constellations from touching the sea. They were forced to continuously circle the sky without rest. We know those constellations as Ursa Major and Ursa Minor, or the big and little dippers.
3. Hyperion: the Titan god of light, wisdom, and vigilance. He married his sister Thea, and they gave birth to Helios, the sun, Selene, the moon, and Eos, the dawn. Hyperion and three of his other brothers, Coeus, Crius, and Iapetus, formed the four pillars that separated and held the heavens above one another. According to one of the more horrifying Greek traditions, the same four pillars pinned their father down while Cronus castrated Uranus with his sickle.
4. Theia: Also called Euryphaessa, she was the Titaness of sight and the shining light of the clear blue sky. She was the wife of Hyperion and mother to Helios, Selene, and Eos. Thea, the goddess of light, was also a ravishing beauty, perhaps the loveliest of the six Titan daughters. She was the goddess of light, and therefore the perfect match for her brother, Hyperion. She also imbued gold, silver, and precious gems with their radiant shine, and spoke through an oracle at Phthiotis in Thessaly.
5. Coeus: The Titan of intellect and the axis of heaven around which the constellations revolved. He was married to his sister Phoebe. Coeus was the keeper of the pillar of the north. He was the Titan god of intellect, and married his sister Phoebe. Their children, Asteria and Leto, were foundational figures in later mythology. Both daughters were pursued by Zeus. Asteria turned into a quail and drowned herself in the Aegean Sea, but Leto bore Zeus two children, the twins Apollo and Artemis who became powerful Olympians.
6. Phoebe: The Titaness of the "bright" intellect and prophecy, and consort of Coeus. She was the grandmother of Apollo and Artemis. Since Phoebe was the grandmother of Apollo and Artemis, the twins’ were sometimes called Phoebus and Phoebe as alternative names. Phoebe also had some association with the moon, as did Artemis. Her most integral power was that of prophecy, and she was heavily associated with the famous Oracle at Delphi, later connected to Apollo.
7. Crius: The least individualized among the Titans, he was associated with the constellation Aries. Crius (or Krios) married his half-sister, Eurybia, who was not one of the original twelve Titans but the daughter of Gaea from her second husband, Pontus. They produced three children, Astraios, the god of dusk, Pallas, god of Warcraft, and Perses, the god of destruction. Crius fought with the Olympians during the Titans’ overthrow, and as a result, he was imprisoned in Tartarus.
8. Themis: The Titaness of divine law and order. She was also a prophecy-giver at Delphi. Themis was the mother of the Fates and the Seasons by Zeus. The Titan goddess Themis represented natural and moral order and law. She became the second wife of Zeus, helped him hold power over the other gods and all the earth. She created the divine laws that even superseded the authority of the gods themselves. She manifested in multiple different forms, and mothered the Fates and the Hours. Themis was the main Titan goddess of the oracle at Delphi, but she was so fond of Apollo that she eventually offered the Oracle to him.
9. Iapetus: Often associated with the underworld, he was the father of Prometheus, Epimetheus, Menoetius, and Atlas by his wife Clymene. The Titan Iapetus was the god of craftsmanship or mortality, varying between sources. He married one of his Oceanid nieces, Clymene, and they produced four sons, Atlas, Prometheus, Epimetheus, and Menoetius. These four sons were the ancestors of the first humans, and each passed a certain detrimental quality onto humanity; brash courage, scheming, stupidity, and violence, respectively.
10. Mnemosyne: The Titaness of memory and remembrance. She was also the mother of the nine Muses by Zeus. The goddess of memory and the voice of the underground Oracle of Trophonios in Boetia, Mnemosyne did not marry one of her brothers but still helped mother the next generation of deities. She slept with Zeus for nine consecutive days, and as a result, gave birth to the nine muses; Calliope, Clio, Erato, Euterpe, Melpomeni, Polymnia, Ourania, Terpsichore and Thalia whose roles were to give artists and philosophers inspiration for creation.
11. Rhea: The wife of Kronos (Cronus) and mother to the first generation of Olympians (Zeus, Hera, Poseidon, Demeter, Hestia, and Hades). Cronus was secure and happy, thinking he had subverted the threat, but Rhea was understandably upset. As the goddess responsible for the flow of Cronus’s kingdom, she was well-placed to interrupt that flow. When she learned she was expecting again, she asked her mother for advice. Gaea helped Rhea conceal her newborn baby, and Rhea swaddled a stone in baby clothes and gave the stone to Cronus to swallow. Cronus was fooled, but Gaea and Rhea carefully concealed little Zeus in a small cave on the island of Crete.
12. Cronus: The youngest and leader of the first Titans. He overthrew his father Uranus and ruled during the mythological Golden Age until he was overthrown by his own son, Zeus. Although he was the youngest son of Gaea and Uranus, Cronus was also the strongest of the Greek Titans. For a brief time, earth enjoyed a Golden Age under his rule. The vices had not yet been invented, and the earth was in total peace and harmony. Yet Cronus did not release his brothers as he had promised, and soon his mother grew angry with him and began to plot his downfall. Cronus learned of a prophecy which stated that, as Cronus had dethroned his father, so one of his children would dethrone him. He therefore took all of his children from his sister and wife, Rhea, as soon as they were born, and swallowed them.
It's important to note that these are the twelve original Titans, but there are many more Titans and Titanesses in Greek mythology, including the offspring of these original twelve