The Great Old Ones are unique, malignant beings of great and immense power. They are worshiped by deranged human cults, as well as by most of the non-human races. The Great Old Ones are currently imprisoned with a few beneath the sea, some inside the Earth, and still others in distant planetary systems (and beyond). The reason for their captivity is not known, though there are two prevailing theories: The first is that they were sequestered by the Elder Gods for committing past transgressions, or the second is that they are sealed off somehow from the rest of the universe by their own volition.
It is believed that the Great Old Ones are intentionally quiescent. To account for this, it is possible that the universe experiences cosmic cycles, similar to the seasons of the earth. Just as some animals hibernate during the winter, so too must the Great Old Ones rest in death-like sleep during the present cosmic cycle. If this is so, the Great Old Ones are currently trapped by powerful cosmic forces and must remain so until such time as "the stars are right"; the event whenupon they are released and can revel once more across the cosmos.
They are a pantheon of demonic extraterrestrial gods that are more known amongst mortals than the Outer Gods but are infinitely less powerful. Regardless of this fact, the Great Old Ones are completely invincible in comparison to humanity and their alien nature is so utterly bizarre that it inspires madness in all who gaze upon them.
Cataclysmic, but not necessarily malevolent, the Great Old Ones have the power to end the world, or even perhaps an entire galaxy, on a whim but don't actively seek the destruction of humanity, perhaps it is due to their vast power; to a Great Old One, humanity is not worth the trouble of extermination.
Of course on the other hand, a Great Old One could simply choose to destroy entire civilizations for no other reason than it felt like it, a Great Old One has no care if its actions cause the deaths of thousands and if a Great Old One should become hungry or bored it sees no wrong in randomly torturing or devouring innocents. According to Lovecraft himself, the Great Old Ones were meant to be amoral rather than malicious, in keeping with his belief that the universe itself was alien and uncaring, but this by no means lessens their danger, arguably it only serves to increase it.