The Old Ones were a race of vastly powerful demons that dominated Earth before humans in the Buffy Universe.
They arrived from another dimension during the creation of Earth and shared the Earth with fellow beings - The Powers That Be. The rise of the Old Ones led the Powers to flee this world, while the demons became rulers of vast territories, commanded enormous armies that worshiped them as deities and constantly made war against each other, turning the Earth into their hell.
They could become as big as they wished to be and were vastly more powerful than even the strongest hybrid demon races, commanding incredible amounts of demonic mystical power. The most powerful Old Ones were also essentially immortal.