Num Lock

Its state affects the function of the numeric keypad commonly located to the right of the main keyboard and is commonly displayed by an LED built into the keyboard.

The Num Lock key exists because earlier 84-key IBM PC keyboards did not have cursor control or arrows separate from the numeric keypad.

Most earlier computer keyboards had different number keys and cursor control keys; however, to reduce cost, IBM chose to combine the two in their early PC keyboards.

On some laptop computers, the Num Lock key is used to convert part of the main keyboard to act as a (slightly skewed) numeric keypad rather than letters.

[1] Since Apple keyboards never had a combination of arrow keys and numeric keypad (but some lacked arrow keys, function keys, and a numeric keypad altogether), Apple has keyboards with a separate numeric keypad but no functional Num Lock key.

A keyboard with Num Lock turned on as indicated by a LED
Numeric keypad
Tilde Exclamation mark At sign Number sign Dollar sign Percent sign Caret Ampersand Asterisk Parenthesis Parenthesis Underscore Plus sign Backspace Backtick 1 (number) 2 (number) 3 (number) 4 (number) 5 (number) 6 (number) 7 (number) 8 (number) 9 (number) 0 Hyphen-minus Equals sign Backspace Tab key Q W E R T Y U I O P Curly bracket Curly bracket Vertical bar Tab key Q W E R T Y U I O P Square bracket Square bracket Backslash Caps lock A S D F G H J K L Colon (punctuation) Quotation mark Enter key Caps lock A S D F G H J K L Semicolon Apostrophe Enter key Shift key Z X C V B N M Bracket Bracket Question mark Shift key Shift key Z X C V B N M Comma (punctuation) Full stop Slash (punctuation) Shift key Control key Windows key Alt key Space bar Alt key thumb Menu key Control key