Wikipedia
Inotify ( inode notify) is a Linux kernel subsystem that acts to extend filesystems to notice changes to the filesystem, and report those changes to applications. It replaces an earlier facility, dnotify, which had similar goals.
Inotify was created by John McCutchan, and it was merged into the Linux kernel mainline in kernel version 2.6.13, released on August 29, 2005; later kernel versions included further improvements. The required library interfaces were added into the GNU C Library (glibc) in its version 2.4, released in March 2006, while the support for inotify was completed in glibc version 2.5, released in September 2006.
One major use is in desktop search utilities like Beagle, where its functionality permits reindexing of changed files without scanning the filesystem for changes every few minutes, which would be very inefficient. By being told directly by the kernel that a file has changed, indexing utilities can achieve change-to-reindexing times of only about a second.
Inotify can also be used to automatically update directory views, reload configuration files, log changes, backup, synchronize, and upload.