Sometimes when my wife works on photos in Photoshop on her Mac, what she gives me back are all the photos I took, with their .jpg extensions in place, as well as a set of "ghost" files.

That is, for every photo, there is an evil twin whose file name is the same ... except that it begins with "._". Also this ghost file are sort of "grayed-out" and don't open when double clicked. I suspect they are hidden files, (I have set windows to show all hidden files) but what find of hidden file -- on that, I'm clueless.

I just delete these files and never have a problem with the photo (.jpg) files, but still I'm dying to know; what are they?

Thanks