Monitor
From ComputerCraft Wiki
The Monitor is a block that can display text on its front side. When several screen blocks are placed on the same plane, it will form a single monitor. It is useful for displaying information at a server spawn, showing a program on the monitor, and even showing the status of a IC2 reactor! (provided you have CcSensors installed)
Recipe
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Using the Monitor
To use a monitor type: monitor <on which side the monitor is(Top,Bottom,Left,Right,Front and Back)> <the name of the program(example "hello")
So, in this case, if you typed "hello" in the name of the program part, the monitor would display "Hello World"