Print screen (often abbreviated Prt Scr, Print Scrn, Prt Scn, or Prnt Scrn) is a key present on most PC keyboards. It is typically situated in the same section as the break key and scroll lock key. The print screen may share the same key as system request.
Original use 
Under command-line based operating systems such as MS-DOS, this key causes the contents of the current text mode screen memory buffer to be copied to the standard printer port, usually LPT1. In essence, whatever was currently on the screen when the key was pressed was printed. Pressing the Ctrl key in combination with Prt Sc turns on and off the "printer echo" feature. When echo is in effect, any conventional text output to the screen will be copied ("echoed") to the printer. There is also a Unicode character for print screen, U+2399 ⎙ print screen symbol.
Modern use 
Newer-generation operating systems using a graphical interface tend to copy a bitmap image of the current screen to their clipboard or comparable storage area, which can be inserted into documents as a screenshot. Some shells allow modification of the exact behavior using modifier keys such as the control key.
In Microsoft Windows, pressing Prt Sc will capture the entire screen, while pressing the Alt key in combination with Prt Sc will capture the currently selected window. The captured image can then be pasted into an editing program such as a word processor or graphics program. Pressing Prt Sc with both the left Alt key and left ⇧ Shift pressed turns on a high contrast mode (this keyboard shortcut can be turned off by the user). Pressing the Ctrl key in combination with Prt Sc usually will capture the entire screen, also. This behavior is there for backward compatibility with users who learned Print Screen actions under operating systems such as MS-DOS.
In GNOME and KDE desktop environments, print screen behavior is similar to that of Microsoft Windows by default. However, a window will additionally pop up, prompting to save the screenshot to a file.
Macintosh does not use a print screen key; instead, key combinations are used that start with ⌘ Cmd+⇧ Shift. These key combinations are used to provide more functionality including the ability to select screen objects. ⌘ Cmd+⇧ Shift+3 captures the whole screen, while ⌘ Cmd+⇧ Shift+4 allows for part of the screen to be selected. Unlike Windows, the standard print screen functions described above save the image to the desktop, rather than to the clipboard. However, using any of the key sequences described above, but additionally pressing the Ctrl will modify the behavior to copy the image to the system clipboard instead.
See also