Right in the middle of the brain. Surrounds the third ventricle. Part of the diencephalon along with the epithalamus and the perithalamus. Relays sensor signals to the cerebral cortex, and helps regulate consciousness, sleep, and alertness.
Also sends sensory information up north to the processor part of the brain. When it's off, while you're asleep, that's why you don't notice noises and stuff.