I have seen many sync clients such as Dropbox that can create "virtual" files that do not really exist on disk. They have a size and you can open them in any program, but they are streamed e.g. from the Dropbox servers.
I don't know how this is called, and I can't find any information about this on the internet. Do I have to write a file system driver for that, or is it possible to do this for example with the Windows API?