Ok, basically closed-angle glaucoma is caused by poor access to the drainage system within the eye. Open-angle glaucoma is when there are problems with the drainage system itself.
For closed-angle, the angle between the iris and the cornea is narrowed (or closed) preventing proper drainage. The fluid can’t even get to the drainage channel (and causes excessive pressure). For open-angle, the angle between the iris and the cornea is open, but the actual drainage channel is blocked somehow.
I’m not a doctor, so I’m sure I’ve simplified things a little…or a lot.
Try this link for pictures: http://www.aafp.org/afp/20030501/1937.html
You will need them for my superficial discussion.
The fluid inside the eye drains from the back to the front and then drains out of the front (anterior) chamber. Sometimes the angle of the iris muscle is such that it blocks fluid from getting into the anterior chamber in the first place. This is closed angle, or narrow angle glaucoma. It’s often a sudden, unilateral, painful, emergent presentation.
Most times this angle is fine, and the fluid gets into the anterior chamber but just is not taken up properly. This is open angle glaucoma, the commonest kind; often silent and progressively damaging.
To some extent people who have a naturally narrow angle of the iris muscle with the wall of the anterior chamber are at higher risk for acute, narrow-angle glaucoma. If something happens such as trauma or drugs that dilates the pupil, the contraction of the iris is enough to close off the drainage and precipitate acute glaucoma.
By itself the angle involved is not an absolute indicator of which kind of glaucoma you can get.