The term 'Autorun', related to computing, refers to any method by which software can be executed automatically without input from a user.
In Microsoft Windows, users are quite familiar with Autorun methods. For example, if you insert a video game disc into a Windows PC (on any Windows operating system up to Windows Vista), you will almost certainly see an installer (or the game itself) launch immediately without having to manually launch it. In this case, Windows Explorer simply reads the contents of the Autorun.inf file on the root directory of the disc. This is a plaintext file that gives instructions on what to execute on the disc.
To avoid this type of Autorun starting, all you have to do is hold down the ESC key when you insert the disc until it has fully spun up.
While this provides convenience for end users, it also became something of a security problem. With versions of Windows from XP downward, the same Autorun.inf file would lead to automatic execution of a specified file on a removable USB key. Malware authors saw the potential in this mechanism to further spread their malicious software. Common malware will now automatically copy itself to a removable drive, and write its own Autorun.inf file so that it will be automatically executed when inserted into a new PC.
The infamous Stuxnet cyberweapon that targeted Iranian nuclear fuel enrichment facilities used Autorun and known Windows flaws to propagate in environments where an Internet connection is not available.
In Windows 7 (and in Windows Vista after a Microsoft Update), this removable media Autorun system was changed so that users are given an option whether or not to execute the file that Autorun.inf mandates.
Autorun can also refer to other ways in which software can execute automatically on a system without user interaction. For example, there are registry keys whose values can be changed to automatically start programs when, say, Windows boots, or when Windows is shutting down. Using the "bootexecute" key, you can even dictate that a certain program launch before Windows has fully loaded. You can see all of these Autoruns on your system by using a tool from Sysinternals (now Microsoft) called Autoruns.
For More Information: Analyze all Autorun / Auto-start programs in Windows