An autorun.inf file is basically a text file that can be used by the AutoRun and AutoPlay components of Microsoft Windows Operating systems. It’s can be associated with any of the drives (USB drive, CD/DVD drive or Hard Disk drive volume) For the file to be in action, it must be located in the root directory of a volume. 
The Autorun.inf file defines the following:
The application or process that will automatically run when a CD/DVD, USB disk or Volume is mounted.
The icon that will represent your CD/DVD, USB disk or any Hard disk Volume when the drive is viewed with My Computer or Explorer.
Menu commands displayed when the user right-clicks the CD/DVD, USB disk or Volume icon from My Computer or Explorer.
A simple Autorun.inf file have the following format :
[autorun]
open=autorun.exe
icon=autorun.ico
Suppose you have a setup.exe file in your USB Pen drive and you want to run this application automatically when you insert your Pen drive in any computer.For this you have to create an autorun.inf file in root directory of your pendrive.
- Open Notepad.
- Write,
[autorun]
open=setup.exe
3. Save the file as autorun.inf in the root directory of your Pendrive in which the setup.exe file is present.
4. Now remove your Pendrive and again insert in to computer you will see the application setup.exe Executed automatically.
Note:
By default, the system looks for the file in the root directory of the inserted disk. If you want to access a file located in a specific folder or subdirectory, specify a path relative to the root.
As Windows has a case-insensitive view of filenames, the autorun.inf file can be stored as AutoRun.Inf or autoRun.inF or any other case combination.
If you want to assign an icon to your Pen drive and also a name for your pen drive then just write two line more after the line open=setup.exe.
icon=youricon.icon
label=yourlabel
Where youricon.icon is the name of your icon file. Valid file types containing icons include .ICO, .BMP and also .DLL and .EXE can contain the icons. If the file includes more than one icon, by default, the second icon in the files icon resource will be displayed. You can write any name instead of yourlabel for your pen drive.
For CD/DVD drive you have to write this autorun.inf file in your CD/DVD.
