Spawner

The Spawner, or Monster Spawner as it is referred to on Bedrock Edition, is able to spawn mobs of a particular type making it fairly useful for farms and other mechanics that require an abundance of mobs. The Spawner is obtainable as an item in both editions of the game and can be modified with in the Java Edition of game. With the Spawner being a block entity there are a variety of discontinued nbt variants obtainable.

Java Edition
Spawner as an item can be used to create a spawner any where in your world.

Bedrock Edition
Monster Spawner as an item can be used to create a spawner any where in your world.