Open angle (wide angle) - small drainage canals get clogged slowly over a long time (chronic). When enough canals are blocked aqueous outflow is impeded and visual disturbances emerge due to increased intraoccular pressure. Treatment with most drugs is Okay.
Closed angle (narrow angle) - the whole outflow tract gets clogged acutely by adhesion of the iris to the cornea (there are probably other mechanisms but this one makes most sence to me and seems to work for most questions). This type of glaucoma presents with pain due to the rapid build up of intraoccular pressure and it is a medical emergency. Drugs that dilate the pupil (mydriasis) cause the iris to swell up around the edges (kinda makes sence if you picture it, you gotta make the center hole bigger so all that extra iris stuff has got to bundle up on the perimiter) and cause it to push up against the cornea even harder, further closing off the outlfow tract. These drugs are contraindicated in closed angle.
Hope that helps.