"If you've never dabbled with filters and pipes before, or perhaps you've just been too scared, we want to help you out, so read on to learn how you can make powerful Linux commands just by stringing smaller bits together...
"A filter is a program that reads a single input stream, transforms it in some way, and writes the result to a single output stream, as shown in Figure 1, below. By default, the output stream (called standard output or just stdout) is connected to the terminal window that the program is running in, and the input stream (standard input, or just stdin) is connected to the keyboard, though in practice filters are rarely used to process data typed in manually at the keyboard."
