In humans, equilibrioception is mainly sensed by the detection of acceleration, which occurs in the vestibular system. Other senses play roles as well, e.g. the visual system and proprioception. For instance, it is harder to stand on one foot with eyes closed than with eyes open.
When the sense of balance is interrupted it causes dizziness, disorientation and nausea. Balance can be upset by Meniere's disease, an inner ear infection, by a bad common cold affecting the head or a number of other medical conditions. It can also be temporarily disturbed by rapid and vigorous movement, for example riding on a merry-go-round. See also vertigo.
Most astronauts find that their sense of balance is impaired when in orbit, because they are in a constant state of free-fall while their rockets are off. This causes a form of motion sickness called space sickness.
Equilibrioception in many marine animals is done with an entirely different organ, the statocyst, which detects the position of tiny calcareous stones to determine which way is "up".