The eye is a sphere filled with fluid. This fluid is produced constantly, and recycled through a drainage system. In some animals the drainage system becomes impaired, while the fluid production continues. This is akin to a balloon becoming filled with water, causing increased pressure on the walls of the balloon. Similarly in the eye, increasing fluid inside the eye increases the pressure, causing pain and ultimately, loss of vision.
Determining a diagnosis of glaucoma requires measurement of the pressure by means of specialized instruments. Glaucoma is incurable, but treatable with drugs to reduce the production of fluid. Glaucoma is sometimes responsive to medical management, but surgery (removal of the affected eye) is often recommended because glaucoma is a chronically painful disease.