Glaucoma is a disease of the optic nerve, which transmits the images you see from the eye to the brain. The optic nerve is made up of many nerve fibers (like an electric cable with its numerous wires). Glaucoma damages nerve fibers, which can cause blind spots and vision loss.
Glaucoma is related to the pressure inside the eye, known as the intraocular pressure (IOP). When the aqueous humor (a clear liquid that normally flows in and out of the eye) cannot drain properly, pressure builds up in the eye. The resulting increase in IOP can damage the optic nerve and lead to vision loss.
There are a number of diseases that affect the retina, the layer of nerve cells lining the back of the eye to sense light and create impulses to the brain where they are recognized as an image.