Many times people confuse it with a cataract which is also a painless, gradual loss of vision. The major difference is that in the case of a cataract, the loss of vision is completely recoverable using a simple surgery called Phacoemulsification. Our eyes contain a clear liquid termed aqueous humor, which is continuously produced in the eye to bathe and nourish the structures inside it.
The liquid usually drains out of the eye through drainage canals in a fine meshwork found around the edge of the iris (the coloured part of the eye that surrounds the pupil). In people with it, the liquid fails to drain due to some defect and thus raises the pressure inside the eyes called raised Intraocular Pressure (IOP) (or Tension).