Union mounts for Linux.
There, wasn't that helpful?
Just kidding: So what's a union mount? Imagine you have a directory with files in it, and you then mount some device on that directory. Ordinarily, the original files would no longer be available, but a union mount leaves them visible: you can see both the files from the device you mounted and the files that were originally in the directory you mounted it on.
Yes, Mac has this also.
This mixing raises interesting problems - for example what happens if there are identically named files? I'm not going to go into the details of all this here because it takes a lot of space and it makes my head hurt, but this is where whiteouts come into play. The basic issue is that the "upper" layer is the last thing mounted, and although everything below is visible, if you want to delete something in the lower layer, you really can't; a "whiteout" in the upper layer makes it seem to be deleted.
Union mounts have been available in BSD and Mac OS 10, see http://www.fsl.cs.sunysb.edu/project-unionfs.html for Linux. Interestingly, unionfs allows you to create snapshots.
If you really want to know about whiteouts, go read
A common use is in Live Cd's: the read only cd file system is paired with a writeable file system on a hard drive.
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