in a typical PC keyboard, you have these keys: PrtScn SysRq ScrLk Pause Break. Ever wonder what are they used for?
Note the PrtScn (Print Screen) key, SysRq (System request) key, ScrLk (Scroll Lock), Break keys. These keys are historical relics and are more or less defunct today, except the Print Screen key that is often used for creating screenshots in Microsoft Windows and Linux. (on the Mac, it's done by 【⌘ Cmd+⇧ Shift+3】)
Here's a summary of what these keys are, based on Wikipedia. Note: these keys are not used on the Mac.
【Ctrl+PrtScn】 → In 1990s or earlier, it sends the screen's text into the printer port (LPT1), causing the screen to be printed. (at the time, most monitors can only display text.) Today, 【PrtScn】 key is used in Windows and Linux to do screenshot (copy screen bitmap into the clipboard).
ScrLk (Scroll Lock) → Used to toggle the behavior of arrow keys so that, when ScrLk is on, the up/down arrows scrolls the window. It may still work when in system bootup screen, and is also used in spread sheet to force arrow keys to scroll window instead moving around cells.
SysRq (System request) → This key causes a interrupt to the operating system. It is kinda like the role of today's Control-Alt-Delete on Windows. Some programing applications still use it, to reboot, reset, or enter debug mode ⁖ It is used by Linux kernel developers Magic SysRq key, and some IDEs.
Pause/Break. It is used to interrupt screen output by BIOS. Used for sending a interrupt signal of sorts, somewhat similar to 【Ctrl+c】 on PC and 【⌘ Cmd+.】 on Mac.
(thx to Rob Shinn at g+ discussion.)