The Summon Command

The /summon command is a useful command normally used in maps.

Overview
The /summon command summons any entity in a certain spot.

Syntax
/summon   

Explanation
The entity value is the entity you want to summon. The x, y, and z coordinates is the location of the entity summoned.

Trivia

 * Is very devastating with TNT.
 * Used in maps for mobs that could despawn.