Mob Spawners are blocks with a transparent blue lattice structure found in dungeons. Inside it are flames representing magic and a miniature model of the mob it spawns, continuously spinning and getting faster until the mob spawns in the area around the cage. Unless surrounded by torches, Mobs will never stop spawning from it; this has lead to many contraptions designed to kill whatever drops from it, such as large pits or lava wells, abusing the spawning mechanic.
A Mob Spawner will not spawn further enemies if the room is well lit.
When found inside dungeons naturally, they will either spawn Spiders, Skeletons, or Zombies. While they can be destroyed by a pickaxe or by hand, they can no longer be picked up by the player. When placed by the player, they would only spawn pigs.
Mob spawners can be edited to spawn mob entities. However, mob spawners cannot be modified to spawn slimes. This is likely because slimes have a size variable.