Printf format string refers to a control parameter used by a class of functions in the input/output libraries of C.
Formatting takes place via placeholders within the format string. For example, if a program wanted to print out a person's age, it could present the output by prefixing it with "Your age is ". To denote that we want the integer for the age to be shown immediately after that message, we may use the format string:
$ printf("Your age is %d", age);