Caps Lock is a lock key on many computer keyboards. Pressing it sets an input mode in which typed letters are uppercase by default (i.e. in All caps). The keyboard remains in caps lock mode until the key is pressed again. On some computers, holding down the shift key while caps lock is on temporarily switches to lowercase.
User interfaces 
Keyboards often include a small LED to indicate that caps lock is active—either on the key itself, or in a row with scroll lock and num lock indicators. However, some new laptop and wireless desktop keyboards lack the LED, instead providing software that gives an on-screen indicator.
People typing case-sensitive passwords that are not displayed verbatim on the screen may not realize that caps lock is on, causing errors. Help guides, tech support materials, and sometimes the interface itself may include advice on checking caps lock before typing a password. In Windows login screens, a warning that caps lock is on is shown in a balloon near the field. In Mac OS X, when caps lock is on, a caps lock symbol (⇪) is displayed inside a password field. Operating systems may also provide audible notifications when caps-lock, num-lock, or scroll-lock buttons are toggled.
Some manufacturers include an option in the controller software to deactivate the caps lock key. This behaviour allows users to decide themselves whether they want to use the key, or to disable it to prevent accidental activation.
The caps lock key is a modified version of the shift lock key that occupies the same position on the keyboards of mechanical typewriters. An early innovation in mechanical typewriters was the introduction of a second character on each typebar, thereby doubling the number of characters that could be typed, using the same number of keys. The second character was positioned above the first on the face of each typebar, and the shift key caused the entire type apparatus to move, physically shifting the positioning of the typebars relative to the ink ribbon. Just as in modern computer keyboards, the shifted position was used to produce capitals and secondary characters.
Because the shift key mechanism on a mechanical typewriter requires more force to operate and is usually operated by the little finger on the left hand, it was difficult to hold the shift down for more than two or three consecutive strokes, therefore the introduction of the caps lock key also helped out people with disabilities who could not hold down more than one key at a time. The shift lock key was introduced so the shift operation could be maintained indefinitely without continuous effort. It mechanically locked the typebars in the shifted position, causing the upper character to be typed upon pressing any key.
The caps lock key on modern QWERTY keyboards differs from the shift lock key in that it capitalizes letters but does not affect other keys, such as numbers or punctuation. Some early computer keyboards, such as the Commodore 64, had a shift lock but no caps lock; others, such as the BBC Micro, had both, only one of which could be enabled at a time.
A version of caps lock that behaves like a traditional shift lock does exist on certain layouts such as the French AZERTY. Some operating systems and window managers allow caps lock to be used for a similar function. This behavior of the caps lock survives, however, in German and Austrian QWERTZ keyboards.
The keyboards of many early computer terminals, including the Teletype Model 33 ASR and Lear-Siegler ADM-3A, the Apple II, and early models of the IBM PC, positioned the Control key on the left of the keyboard, where caps lock resides on most modern keyboards. This layout was preserved for later workstation systems and is often associated with Unix workstations. Keyboards from Sun Microsystems came in two layouts; "Unix" and "PC-style", with the Unix layout having the traditional placing of the Control key and other keys. The keyboards produced for One Laptop Per Child computers also have the Control key in this location. Other vendors produce keyboards for different computer systems with this placement of the Control key, such as the Happy Hacking Keyboard. In a similar vein, the Google Cr-48 netbook replaces caps lock with a dedicated "Search" key.
Some users of keyboards with caps lock on the left remap the keys to exchange Control and caps lock, finding the traditional location more ergonomic for using programs benefiting from use of the Control key. Keyboard layout preferences specifically to address this need are available in some operating systems.
International Caps Lock Day 
June 28th and October 22nd are annually marked as International CAPS LOCK DAYS as a parody holiday first created in 2000 by Derek Arnold, a user on Metafilter. Rivaling Caps Lock Days have been since set up, such as August 22nd.
- Sun hardware reference manual http://docs.oracle.com/cd/E19683-01/806-6642/806-6642.pdf
- Don Marti (2006-10-27). "Doing it for the kids, man: Children's laptop inspires open source projects". Linux World. Retrieved 2010-06-11.
- Diaz, Jesus (December 7, 2010). "Google wants to take your Caps Lock key away". Gizmodo. Retrieved December 9, 2010.
- Brian, Matt (October 22, 2010). "Today is INTERNATIONAL CAPS LOCK DAY, but what is it?". Retrieved October 22, 2010.
- Blass, Evan (October 22, 2006). "HAPPY INTERNATIONAL CAPS LOCK DAY". The Next Web. Retrieved October 22, 2010.
- "August Caps Lock Day". August 22, 2011. Retrieved August 22, 2011.
- Wiki:CapsLockOff—describes how to remap the caps lock key for other purposes.
- CAPSoff Official Website
- Caps lock day official site.