2porscheguy gave you some good basic information.
Another form that can happen quickly and progress rapidly is neovascular glaucoma where blood vessels grow on the iris and into the angle where fluid normally drains. People with Diabetes that is not well controlled or for many years as well as people who have had vascular occlusions in the eye are at risk for this form. This can be particularly painful and difficult to control with a high risk of substantial vision loss.
A very nice summary FAQ is available at:
Frequently Asked Questions — American Glaucoma Society
Well written without being too technical.