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There are many diverse causes potentially induce glaucoma. The common denominator of all is the increased intraocular pressure level that is above the optic nerve's "tolerance" limits. These limits are not the same for everyone, resulting in pressures that are considered "normal" for some, to be harmful for others.
A certain level (normal) pressure is necessary for the eye so that it retains its shape and functionality. The increase, however, either by direct effect on the optic nerve, or by compressing the small blood vessels that nourish, causes irreversible damage to the optic nerve with devastating visual consequences.
A fluid called aqueous humor is responsible for intraocular pressure. The aqueous humor is continually produced. In other words, new aqueous humor is constantly produced from the eye, whilst the equivalent amount of aqueous humor is removed by means of a drainage system which we refer to as the "angle". The angle is located in the periphery of the iris, the colored part of that eye and the junction with the inner surface of the cornea.
If the drainage system (angle) is obstructed for any reason, then the produced fluid cannot be removed resulting in its accumulation and increase of eye pressure. Nevertheless, with aging, the drainage capacity of the angle is decreased, and this is why people over 40 years must also be tested for intraocular pressure during their eye examinations.
Ocular hypertension
Just as "normal" intraocular pressure values can cause glaucoma in some patients, some people can tolerate high levels of pressure without this causing visible damage to the optic nerve. This condition is called "ocular hypertension" and requires close observation for any transition to symptomatic disease, although it may need to be preventively treated with medication.