C# programs perform I/O through streams.
A stream is linked to a physical device by the C# I/O system.
The I/O classes and methods can be applied to many types of devices.
At the lowest level, all C# I/O systems operate on bytes.
The Predefined Streams
Console.Out: the standard output stream.
Console.In: standard input, which is by default the keyboard.
Console.Error: the standard error stream, which is also the console by default.