Check out this article:

http://www.linuxjournal.com/article/8478

http://lwn.net/Articles/104343/

Enter "man inotify" at the Linux command line:

NAME
       inotify - monitoring file system events

DESCRIPTION
       The  inotify  API  provides  a mechanism for monitoring file system events.  Inotify can be used to monitor individual files, or to monitor directories.
       When a directory is monitored, inotify will return events for the directory itself, and for files inside the directory.


And this article:

Documentation/filesystems/inotify.txt