The history of greek gods is very long and sometimes tedious. In very early Greek times, they worshipped statues and such, but later they developed gods and goddesses. There are many, but the most known are the 12 main Olympian gods, listed below:
Zeus - God of gods, womanizer, powerful symbol - lightning bolt
Hera - Queen of gods, Zeus' wife, jealous, goddess of women and childbearing symbol - peacock
Hestia - Godess of the hearth symbol - flame
Poseidon - God of the sea, easily angred symbol - trident
Athena - Goddess of wisdom and the arts. Is Zeus' daughter symbol - owl
Apollo - God of light, music, the sun, medicine. symbol - lyre
Artemis - goddess of the moon and the hunt. Apollo's twin sister. symbol - bow and arrow
Aphrodite - Goddess of love and beauty, married to Hephestus symbol - swan
Hephestus - God of metalworking, very kind but ugly symbol - hammer
Demeter - Goddess of agriculture, nice, mother of Persephone symbol - wheat
Ares - God of war, brutal, mean symbol - spear
Hades - God of the underworld, cruel, unhappy symbol - Cerberus, his 3-headed dog
Dionysus - God of wine and celebration, cheerful symbol -grapes
Hermes - messenger god, trickster symbol - his shoes
There is SOOO much more to Grecian mythology than that. Each person has their own story, and it's important to know about their backgrounds to fully understand why they do what they do. It'll also explain why there are fourteen Olympian gods instead of 12!