Command Block

A command block is an indestructible block that can execute commands. Because it cannot be obtained, destroyed or used in Survival mode without cheats, it is primarily used in multiplayer servers, Creative worlds, and custom maps. There are 3 types of these blocks.
An impulse command block is the default block type; it executes only once when activated.
A chain command block executes every time when triggered.
A repeating command block executes every game tick as long as it is activated.