It's when you tell a function to call another function (specified by you) at some point.

#include <stdio.h>

static void count_to_five(int (*print_cb)(const char *))
{
   char str[4];
   int i;

   for (i = 0; i < 5; i++) {
      snprintf(str, sizeof(str), "%d", i);
      (void)print_cb(str);
   }
}

static int custom_puts(const char *str)
{
   printf("foo: %s\n", str);
   return 0;
}

int main(void)
{
   count_to_five(puts);        /* use libc puts() function to print numbers */
   count_to_five(custom_puts); /* use our custom_puts() function to print numbers */
   return 0;
}

In some cases you define a callback to use as a handler for some kind of event. For example, using GTK, you can define a callback function that will be executed when a certain object is clicked.

HTH

Cheers

A callback function is a function which you register/ask to be called later, usually as a result of a particular event happening.

You register a function to be called back on a particular event, and may also pass in some data with the registration. When the event occurs the function is called and the data you provided when registering comes in as a parameter (which may be used to determine what the function does, so you can register the same function with different data for different events and get it to do different things).

It's used a lot in X Windows programming. I suspect device drivers can be looked at the same way (you register an "open", "read", etc. handler).

http://www.codeguru.com/cpp/cpp/cpp_mfc/callbacks/article.php/c10557#more