The Titans, also known as the elder gods, ruled the earth before the Olympians overthrew them. The ruler of the Titans was Cronus who was de-throned by his son Zeus. Most of the Titans fought with Cronus against Zeus and were punished by being banished to Tartarus.
Gaea
Gaea was Mother Earth. She mated with her son Uranus to produce the remaining Titans. Gaea seemed to have started as a neolithic earth-mother worshipped before the Indo-European invasion that eventually lead to the Hellenistic civilization.
Uranus
Uranus was the sky god and first ruler. He was the son of Gaea, who created him without help. He then became the husband of Gaea and together they had many offspring, including the Cyclopes, the Hecatoncheires, and twelve of the Titans.
His rule ended when when Cronus, encouraged by Gaea, castrated him. He either died from the wound or withdrew from earth.
Cronus
Cronus was the ruling Titan who came to power by castrating his Father Uranus. His wife was Rhea. Their offspring were the first of the Olympians. To insure his safety, Cronus ate each of the children as they were born. This worked until Rhea, unhappy at the loss of her children, tricked Cronus into swallowing a rock, instead of Zeus. When he grew up, Zeus would revolt against Cronus and the other Titans, defeat them, and banish them to Tartarus in the underworld.
Cronus managed to escape to Italy, where he ruled as Saturn. The period of his rule was said to be the golden age on earth, a time of peace and happiness that was honored by the Saturnalia feast. During the golden age, the people of the time had no need for laws or rules; everyone did right and as such, there was no need.
Rhea
Rhea was the wife of Cronus. She was the Titan of the earth and fertility. Cronus made it a practice to swallow their children. To avoid this, Rhea tricked Cronus into swallowing a rock, saving her son Zeus.
Rhea's symbol is the moon. She has another symbol, the swan, because it is a gentle animal. Also, her other symbol is two lions, supposedly the ones that pull her chariot.
Oceanus
Oceanus was the Latin word for the ocean, which the Greeks and Romans believed to be an enormous river encircling the world. Strictly speaking, it was the ocean-stream at the Equator in which floated the habitable hemisphere. This world-ocean was personified as a Titan, a son of Uranus and Gaea. Together with his wife Tethys, they produced the rivers and six thousand offsprings called the Oceanids.
Tethys
Tethys was the wife of Oceanus. She was mother of the chief rivers of the universe, such as the Nile, the Alpheus, the Maeander, and about six thousand daughters called the Oceanids.
Hyperion
Hyperion was the Titan of light, an early sun god. He was the son of Gaea and Uranus. He married his sister Thea. Their children were Helius (the sun), Selene (the moon), and Eos (the dawn).
Mnemosyne
Mnemosyne was the Titan of memory. She slept with Zeus for nine nights and gave birth to the nine Muses.
Themis
Themis was the Titan. Her name meaning "law of nature" rather than "human ordinance" was "of good counsel," was the embodiment of divine order, law and custom. She fought with Zeus against the other Titans. She was the mother of the Fates and the Horae (Seasons).
Iapetus
Iapetus was the father of Prometheus, Epimetheus, Menoetius, and Atlas by Clymene.
Coeus
Coeus was the Titan of Intelligence. He was the father of Leto and Asteria.
Cruis
Cruis married his sister Eurybia and became the father of Astraeus, Pallas and Perses.
Phoebe
Phoebe was Titan of the Moon. She was the mother of Leto and Asteria.
Thea
She, along with her brother Hyperion, was the mother of Helios, Selene and Eos. She seems here a goddess of glittering in particular and of glory in general.
Prometheus
Prometheus was the wisest Titan. His name meant "forethought" and he was able to foretell the future. He was the son of Iapetus. When Zeus revolted against Cronus, Prometheus deserted the other Titans and fought on Zeus' side.
By some accounts, he and his brother Epimetheus were delegated by Zeus to create man. In all accounts, Prometheus was known as the protector and benefactor of man. He gave mankind a number of gifts including fire. He also tricked Zeus into allowing man to keep the best part of the animals sacrificed to the gods and to give the gods the worst parts.
For this Zeus punished Prometheus by having him chained to a rock with an eagle tearing at his liver. He was to be left there for all eternity or until he agreed to disclose to Zeus which of Zeus' children would try to replace him. He was eventually rescued by Hercules without giving in to Zeus.
Epimetheus
Epimetheus was a stupid Titan, whose name meant "afterthought". He was the son of Iapetus. In some accounts, he was delegated, along with his brother Prometheus by Zeus to create mankind. He also accepted the gift of Pandora from Zeus, which lead to the introduction of evil into the world.
Atlas
Atlas was the son of Iapetus. Unlike his brothers Prometheus and Epimetheus, Atlas fought with the other Titans supporting Cronus against Zeus. Due to Cronus's advance age, Atlas led the Titan's in battle. As a result, he was singled out by Zeus for a special punishment and was forced to hold up the world on his back.
Metis
Metis was the Titaness of the forth day and the planet Mercury. She presided over all wisdom and knowledge. She was seduced by Zeus and became pregnant with Athena. Zeus became concerned over prophecies that her second child would replace Zeus. To avoid this Zeus ate her. It was said that she was the source for Zeus' wisdom and that she still advises Zeus from his belly.
It may seem odd for Metis to have been pregnant with Athena but, never mentioned as her mother. This was because the classic Greeks believed that children were generated solely from the father's sperm. The women was thought to be nothing more than a vessel for the fetus to grow in. Since Metis was killed well before Athena's birth, her role doesn't count.
Dione
According to Homer in the Iliad, she is the mother of Aphrodite.