Glaucoma is caused by a number of different eye diseases, which in most cases produce increased pressure within the eye. In an eye that has glaucoma, more fluid is produced, than can be removed by the eye. With nowhere else to go, this fluid builds up in the front of the eye, causing an increase in pressure.
Over time, this increase in pressure transfers to the rest of the eye. The optic nerve, the weakest area of the eye, is most vulnerable to damage from this elevated pressure. Continuous elevated pressure, or spikes in pressure can damage the optic nerve, which may result in vision loss and can also lead to blindness if left untreated.