Greek Gods and Goddesses:
There are many types of immortal beings in Greek mythology. Some are depicted as humanoid, some as part animal, and some personifications are not readily visualized. The gods and goddesses of Mt. Olympus can walk among mortals undetected. They each tend to have a special area they control. Thus, you have the god of thunder or grain or the hearth.
In Greek Gods and Goddesses you'll find information on the 12 Olympians, the children of the Titans Cronus and Rhea, as well as some of the other Titans, who are the children of Gaia and Uranus (Earth and Sky) and their non-Olympian children.
Individual Gods and Goddesses From Mt. Olympus:
The Titans are among the more confusing of the immortals of Greek mythology. Some of them are stuck in the Underworld suffering for their misdeeds against the Olympian gods. There are two important generations of Titans.
Myths and Legends tells some of the major stories about the ancient immortals.
Special Female Deities - Muses and Nymphs:
The Muses were considered responsible for the arts, sciences, and poetry and were the children of Zeus and Mnemosyne, born in Pieria. Here you'll find pictures of them, their spheres of influence, and their attributes:
Nymphs appear as beautiful young women. There are several types and some individual nymphs who are famous in their own right.
Naiads are one variety of nymphs.
Roman Gods and Goddesses:
When talking about Greek mythology, the Romans are usually included. Although their origins may have been different, the main Olympian gods are the same (with a name change) for the Romans.
Even before the Romans started expanding their empire around the time of the Punic Wars, they came in contact with other native peoples in the Italic peninsula. These had their own beliefs, many of which influenced the Romans. The Etruscans were particularly important.
Gregory Flood's List of Roman Gods and Goddesses shows the spirits and personifications.
Greek mythology has animal and part animal creatures. Many of these have supernatural powers. Some, like the Centaur Chiron, are capable of giving up the gift of immortality. Others can be killed with great difficulty and only by the greatest of the heroes. Snake-haired Medusa, for instance, killed by Perseus aided by Athena, Hades, and Hermes, is one of the 3 Gorgon sisters and is the only one who can be killed. Perhaps they don't belong in a grouping of immortals, but they aren't quite mortal, either.
There were many beliefs in the ancient world. When the Romans started expanding, they sometimes joined together native deities with ones that sounded similar from back home. In addition to the religions with many gods there were others like Judaism, Christianity and Mithraism that were basically monothesistic or dualistic.
Here are some articles on mythology and beliefs in general, and on specialized topics, like oracles and writers about Greek mythology:
Greek Mythology Quiz:
Take a mythology quiz. There are 10 questions in this self-grading Greek mythology quiz covering more than just the immortals.
Greek Mythology Study Guide:
The stories of Greek mythology include myths about the origin of the world, the creation of humans, the bringing of fire to mankind, a great flood, and more. Greek myth is not as organized a set of beliefs as modern monotheistic ones, so the Study Guide also looks at what is meant by Myth and how it differs from related items. Some of the topics covered are