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