The gods of ancient Greece, most of whom were adopted by the ancient Romans, were generally described as human in form, unaging, nearly immune to all wounds and sickness, capable of becoming invisible, able to travel vast distances almost instantly, and able to speak through human beings with or without their knowledge. In Greek mythology, the gods were presented as a large, multi-generational family, the oldest members of which created the world as we know it.
Each Greco-Roman divinity has his or her own specific appearance, genaeology, interests, personality, and area of expertise, subject to significant local variants. When the gods were called upon in poetry or prayer, they were referred to by a combination of their name and epithets, the latter serving to distinguish them from other gods.
The deities of ancient Rome were based on the Greek pantheon, but also included gods and goddesses incorporated from other cultures encountered and conquered by the Roman Empire (such as Egyptian and Persian) and divinities associated with the Roman state.
The Olympian Gods
The Twelve Olympians were the principal gods of the Greek pantheon, residing atop Mount Olympus. There were fourteen different gods recognized at some point as Olympians, though never more than twelve at one time. Zeus, Hera, Poseidon, Ares, Hermes, Hephaestus, Aphrodite, Athena, Apollo, and Artemis were always considered Olympians.
Hestia, Demeter, Dionysus, and Hades are variable gods among the Twelve. Hestia gave up her position as an Olympian to Dionysus in order to live among mankind (eventually she was assigned the role of tending the fire on Mount Olympus). Demeter was allowed to leave Olympus six months of the year to be with her daughter Persephone in Hades (causing winter). And, although Hades was always one of the principal Greek gods, his home in the underworld of the dead made his connection to the Olympians more tenuous.
The Olympians gained their supremacy in the world of gods after Zeus led his siblings to victory in war with the Titans; Zeus, Hera, Poseidon, Demeter, Hestia, and Hades were siblings; all other Olympians (with the usual exception of foam-born Aphrodite) are usually considered the children of Zeus by various mothers (except for Athena, who was possibly born of Zeus alone). Additionally, it is also possible that Hephaestus was born of Hera alone as Hera's revenge for Zeus's solo birth of Athena.
Aphrodite
Aphrodite is the goddess of romantic love, sexuality and procreation. She is also associated with vegetation, illustrating the strong association between human fertility and fruitfulness of the land. Girls about to be married often sacrificed to Aphrodite so their first sexual experience would be propitious, and Aphrodite was the protectress of prostitutes in Athens and Corinth. Men also worshipped Aphrodite in her role as goddess of seafaring and magistrates. Tales of Aphrodite's origins vary: Hesiod says she was born from the severed genitals of Uranus, while Homer describes her as the daughter of Zeus and Dione.
Apollo
Apollo is the son of Zeus and Leto and brother of Artemis. He is considered the most "Greek" of all Greek gods, and presides over poetry, music, healing, purification, prophecy and the care of young citizens. He also provided prophecies and oracles at his shrines at Delphi and elsewhere. In Greek art, Apollo is depicted as a young, beardless, athletic ideal of male beauty. His weapon is the bow and his plant is the laurel.
Ares
Ares is the son of Zeus and Hera and the god of war. He often represents more negative aspects of war than Athena. In Homer's Iliad, Ares is insatiable, hated by Zeus, attended by his sons (with Aphrodite) Fear and Panic, and fights on the side of the Trojans. Ares was originally represented in iconography as a bearded warrior, but was later shown naked and young. Cults and rituals associated with Ares are rare, though he had temples in Crete and the Peloponnese.
Artemis
Artemis is the elder twin sister of Apollo and daughter of Zeus and Leto. She is a virgin huntress who presides over girls' transitions into women and over childbirth and child rearing. Artemis is generally also associated with the wild and young animals; in imagery, she usually carries a bow and arrow and is shown with a deer (as in this statue, displayed at the Louvre Museum). Artemis was also associated with male rites of passage, hunting and some aspects of war. Like Apollo, she sometimes brought sudden death and punishment.
Athena
Athena is the Greek goddess of wisdom, strategy, and war associated by the Romans with their Etruscan goddess Minerva. She is attended by an owl, carries the goatskin shield (Aegis) given to her from her father and is accompanied by the goddess of victory, Nike. Athena is an armed warrior goddess, never a child, always a virgin (parthenos). The Parthenon at Athens, Greece is her most famous shrine. She never had a consort or lover.
Dionysus
Dionysus (or Dionysos; also known as Bacchus in Roman mythology and associated with the Italic Liber), the Thracian god of wine, represents not only the intoxicating power of wine but also its social and beneficent influences. He is viewed as the promoter of civilization, a lawgiver, and lover of peace — as well as the patron deity of both agriculture and the theater. Within the Greek Olympian tradition he is made to be the son of Zeus and Semele; other versions of the story contend that he is the son of Zeus and Persephone.
Hephaestus
Hephaestus (also Hêphaistos or Hephaestos) is the Greek god of fire and the forge. His approximate Roman equivalent is Vulcan. Hephaestus is the god of blacksmiths, craftsmen, artisans, sculptors, metals and metallurgy and fire. He was worshipped in all the manufacturing and industrial centers of Greece, especially Athens. Though his forge lay in the volcanic heart of Lemnos, Hephaestus became associated with Mount Etna by Greek colonists in Sicily.
Hera
Hera the Great Goddess of pre-Hellene Minoan culture was transmitted to the Greeks as the wife and sister of Zeus. She then presided as goddess of marriage. She had a long separate existence before she was incorporated, with considerable difficulty, into the pantheon dominated by Zeus. In late anecdotal versions of the myths she appeared to spend most of her time plotting revenge on the other women her husband consorted with. Hera was always faithful, and was portrayed as majestic and solemn. She was called Juno by the Romans.
Hermes
Hermēs (Greek "pile of marker stones") is the god of boundaries and of the travelers who cross them, of shepherds and cowherds, of orators, literature and poets, of athletics, of weights and measures and invention and commerce in general, of the cunning of thieves, and the messenger from the gods to humans. Hermes gives us our word "hermeneutics" for the art of interpreting hidden meaning. Hermes and Dionysus are the youngest of the Olympian pantheon. The son of Zeus and a primordial nymph named Maia, Hermes was born in a cave on Mt. Cyllene in Peloponnesus, between Achaia and Arcadia. The Romans knew Hermes as Mercury.
Hestia
The virginal Hestia is the goddess of the hearth, of the right ordering of domesticity and the family. She received the first offering at every sacrifice in the household, but had no public cult. In Roman mythology her approximate equivalent was Vesta, who personified the public hearth, and whose cult round the ever-burning hearth bound Romans together in the form of an extended family.
Poseidon
In Greek Mythology, Poseidon was the god of the sea, known to the Romans as Neptune, and to the Etruscans as Nethuns. He was also the god of earthquakes and horses. Like Dionysus and the Korybantes, Poseidon also caused certain forms of mental disturbance. One Hippocratic text says that he was blamed for certain types of epilepsy. Sailors prayed to Poseidon for a safe voyage, sometimes drowning horses as a sacrifice.
Zeus
Zeús Krónios (descendant of Cronus), or simply Zeús (Greek Ζεύς) or Dias (Greek Δίας) ("divine king") is the leader of the gods and god of the sky and thunder in Greek mythology. Zeus was brother and husband of Hera. Their children were Hephaistos, Eileithyia, Hebe and Ares. Zeus is famous for his many extramarital affairs with various goddesses — notably Demeter, Latona, Dione and Maia — and mortal women — notably Semele, Io, Europa and Leda as well as many nymphs.