The iris is blocking the drain. This is why this type of glaucoma is called angle-closure glaucoma. Open-angle glaucoma often comes on slowly and is commonly called chronic glaucoma. Yes, more complex forms of glaucoma exist. One form is combined mechanism glaucoma.
Congenital glaucoma is a disease which affects infants and is extremely rare.
It is an inherited condition. The secondary glaucomas are another category. These forms of glaucoma are ones which are caused by other underlying eye diseases, such as eye inflammation, tumor, retinal detachment, diabetic eye disease, etc. An additional form is low tension glaucoma. In this variety of glaucoma, damage to the optic nerve can occur even at pressures in the eye which are considered normal for most people's eyes.