Caret notation is a notation for unprintable control characters in ASCII encoding. The notation consists of a caret (^) followed by a capital letter; this digraph stands for the ASCII code that has the numerical value equivalent to the letter's numerical value. For example, ^D represents the end-of-transmission character (abbrevated EOT), which cannot ordinarily be shown on the screen. The letter is D, because it is the 4th letter in the alphabet and EOT has the value 4 in the ASCII encoding. The null character (NUL) is represented as ^@ (@ is the ASCII character before A).
For a complete list of caret notation, see C0 and C1 control codes.
Many computer systems allow you to enter a control character by holding down Ctrl and pressing the letter used in the caret notation. This is practical, because many control characters (e.g. EOT) can not be entered directly from a keyboard. Although there are many ways to represent control characters, this correspondence between notation and typing makes the caret notation suitable for many applications.