What is Angband?
It's a freeware computer role playing game based (loosely) on the books of J.R.R.Tolkien. You explore a very deep dungeon, defeat monsters, try to equip yourself with the best weapons and armor you can find, and finally face Morgoth - "The Dark Enemy".
From Thangorodrim, the official Angband Web Site

What is Oangband?
Oangband is one of many variants of Angband, emphasising intuitive rules, varied gameplay, and extra coolness. This long-established variant (one of the older ones active) offers more individual and distinct classes and specializations, more intelligent and diverse monsters, and more subtle and interesting objects. Depth of play is tremendous, with well-balanced ways to defeat monsters that work from the moment you walk into the dungeon, to that dread day you face Morgoth, wielder of the Udun-Fire.