Gods and goddesses are beings of various races who have gained great power. Although they have immense power, they are not invincible; gods, even those such as Zeus, can be killed with a necrosword such as All-Black the Necrosword. A weaker god can also be killed in combat.
The concept of a god has a very broad definition. Although there are some, such as the aforementioned Zeus, who have true immense power, others, such as Loki, are referred to as a god through their modus operandi. Organized religion based on gods does exist, as seen on Earth, although polytheistic beliefs have been tested on Earth due to the existence of Thor. In spite of this, the presence of religion on Earth is still strong, with Matt Murdock being a well-known example of a Catholic and Peter Quill still practicing Christianity despite his father, Ego, being a Celestial. God worship has also been observed on other planets, such as Rapu's planet.
There have been at least a hundred or so gods throughout the universe. Many of them congregate in Omnipotence City.