There is no exact equivalent to Windows reparse points. I'm not familiar with them (I only have a vague idea of what they do), but from what I can tell, they are a sort of hybrid between hard links, mount points and symbolic links.
A hard link is a file that is in multiple locations on the same filesystem. As this is not very useful to most users I don't dwell on them further.
A symbolic link is a special file that says “look over there instead”. All programs that affect the content of the file see the actual underlying file instead. Only programs that manipulate files as directory entries (e.g. renaming files) or give special treatment to symbolic links see the symbolic links as such. You can have symbolic links to any type of files: regular files, directories, etc. A symbolic link is purely textual. The target may or may not exist at any point in time. If the target moves, the symbolic link doesn't follow — the flip side of the coin being that you can transparently replace the target of a symbolic link.
A mount point is a directory at which the filesystem driver changes. For example, if a CD drive is mounted at
/media/cdrom, then the file
/media/cdrom/wibble is the file called
wibble at the root of the CD. For more background and examples, see
There are plenty of filesystems that do not correspond to files on a storage medium. They can be files accessed over the network, pseudo-files served by the kernel, alternate views of other parts of the filesystem, archives seen as directories, and countless other possibilities.
I think that Windows's junction points are closer to symbolic links (but with some tracking of the target), and reparse points are about as general as mount points.
FUSE (Filesystem in Userspace) is a general programming framework to build filesystems where an application has entire control over what is seen as a file. All requests to manipulate files (read, write, list, rename, etc.) on a FUSE filesystems are handled by a user process. In other words, these are precisely “files whose contents when read are not the contents on the actual disk, but whatever the application they request says they are”. There are hundreds of FUSE filesystems around (if not more).
Files from Skydrive appearing as files on a mounted filesystem has been done, with FUSE: skydrive-fuse-fs. I have no idea about the code maturity, the project doesn't look very active, however it has official backing. See also mounting skydrive as a folder and How do I mount a Windows Live SkyDrive account as a partition?.
If you want to mix local files and remote files in the same directory, the general concept is a union mount. You can make a union mount of a local filesystem (or a directory tree in a local filesystem) and a Skydrive filesystem. There are several union filesystems using FUSE, including unionfs-fuse, Unionfs and AUFS.
What a union mount won't give you is automatically making a local copy of files the first time you access them. For that, you need a caching filesystem. Possibilities include pcachefs and MCachefs; I don't know how mature they are.