Minecraft (Pocket Edition)
The above image is of a monster spawner with a torch placed on top
Location / Appearance
Dungeons are a relatively common single-room structure you will encounter when caving. From the outside they can be distinguished by their side walls made from Cobblestone and a floor made of Moss Stone.
Usage / Purpose
Besides containing a single monster spawner of varying types (Zombie and Skeleton are common) they also contain at least one Chest, often two. Dungeon Chests typically contain some of the following inventory items, in varying quantities:
One good sign of a nearby Dungeon is the heavy presence of creatures from the same Mob type. Because there will potentially be a lot of creatures being generated and already existing in the Dungeon, it is important to be prepared for a prolonged battle. This includes having adequate Food, Armor and Swords. A Bow and large supply of Arrows may be useful if it is a Skeleton spawner.
Another useful strategy is to block off any entrance/exit to the Dungeon. Leave one block open at ground level and then mine down a bit on the outside edge of the Dungeon. This will allow you to place adequate lighting in the Dungeon to stop the Monster Spawner from spewing out creatures, while at the same time attacking from a safe position near the creature's feet.
Besides the loot from the Chests, the Dungeons also present an opportunity to collect a large number of drops from whatever creature type was being spawned in the Dungeon.