System.out

In short: System.out is the standard output stream for text on the console in Java.

In more detail: The methods println, print and printf write text.

In Depth

System.out.println("Hello");            // with line break
System.out.printf("%d + %d = %d%n", 2, 3, 5);
System.err.println("Error");            // error stream

System.in reads input (for example with Scanner). See input and output, output, input.

See also: Input and Output (I/O), Output (Output Device), Input (Input Device)