Glaucoma is a disorder caused by increased intraocular pressure damaging the optic nerve. It has two main types - open angle glaucoma where the angle remains open, and angle closure glaucoma where the angle becomes narrow or closed. Angle closure glaucoma occurs when the iris blocks the drainage angle, commonly due to pupillary block or narrowing of the angle with age. It presents with sudden severe eye pain, blurred vision, headache, and eye redness. Gonioscopy is used to examine the angle structure and grade its width.