The command block is a minecraft block designed to be used in Adventure Mode. With a command block a map creator will be able to ‘instruct’ a map/block to dispense specific items and EXP points when the block is activated via a redstone current. This means that when a player completes an objective in adventure mode, they can be rewarded. The command block can also be used to change how the game behaves at different stages of the map. Mob spawning, fire spread, block and mob drops can all be controlled via the command block. The command block can also be used in multiplayer server maps (SMP).
Click here to read more and learn how to use this new minecraft command block!