SysRq, SysReq or Sys Req (short for System Request) is a key on keyboards for PCs that has no standard use. This key can be traced back to the operator interrupt key used on IBM 3270-type console keyboards of the IBM System/370 mainframe computer, which was used to cause the operating system such as VM/370 or MVS to allow the console to give input to the operating system.
The Alt key on an IBM PC keyboard is the key located immediately to either side of the Space bar, used to change (alternate) the function of other pressed keys. Thus, the Alt key is a modifier key, used in a similar fashion to the Shift key. For example, simply pressing "A" will type the letter a, but if you hold down either Alt key while pressing A, the computer will perform an "Alt-A" function, which varies from program to program. In non-US keyboard layouts, rather than a second Alt key, there is an almost identical 'Alt Gr' key to the right of the space bar.
AltGr (Alternate Graphic) is a modifier key on PC keyboards used to type many characters, primarily ones that are unusual for the locale of the keyboard layout, such as foreign currency symbols and accented letters. If a key has a third symbol on it (on the front vertical face or the bottom right of the key top, sometimes in a different colour), then AltGr can often be used to type that character.
The scroll lock key is a remnant from the original IBM PC keyboard. In the original design, scroll lock was intended to modify the behavior of the arrow keys. When the scroll lock mode was on, the arrow keys would scroll the contents of a text window instead of moving the cursor. In this usage, scroll lock is a modifier key like Alt and Shift (which modify the function of other keys) and, more specifically, a toggling lock key like Num Lock or Caps Lock, which have a state that persists after the key is released. Today, this particular use of scroll lock is rare. Only a few modern programs still honor this behavior, such as Microsoft Excel (in the behavior of arrows — when scroll lock is on, the selection does not move), Lotus Notes and Forté Agent.