ioctl stands for input/output control and is used to manipulate a character device via a filedescriptor. The format of ioctl is
ioctl(unsigned int fd, unsigned int request, unsigned long argument).
The return value is -1 if an error occured and a value greater or equal than 0 if the request succeeded just like other system calls. The kernel distinguishes special and regular files. Special files are mainly found in /dev and /proc. They differ from regular files in that way that they hide an interface to a driver and not to a real (regular) file that contains text or binary data. This is the UNIX philosophy and allows to use normal read/write operations on every file. But if you need to do more with a special file or a regular file you can do it with ... yes, ioctl. You more often need ioctl for special files than for regular files, but it's possible to use ioctl on regular files as well.