The Olympians are a group of 12 gods who ruled after the overthrow of the Titans. All the Olympians are related in some way. They are named after their dwelling place Mount Olympus.
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.
The Lesser gods play a role in mythology. While they may not have had the stature of the Olympian Gods, they nonetheless have a role to play in these great stories.
There are many other notable characters that play a role in mythology.