仮想ファイルシステム（Virtual File System）とは？

A virtual filesystem (VFS) is an abstraction layer on top of the actual filesystem implementations (such as the default filesystem for your OS) that allows client applications to interact with these filesystems in a uniform way, using the same APIs -- In other words, applications such as Finder, Microsoft Office, etc. can use the same filesystem APIs standardized by the VFS layer, which then routes these calls to the appropriate filesystem implementations. Filesystems, therefore, have great flexibility on how the data is accessed and presented, as long as they implement a standard VFS interface. While most filesystems interact with a hard disk, the VFS layer allows for filesystems that get data from some other source! Here are some interesting filesystems that have been developed over the years:

SSHFS: Presents files and folders in a user-specified directory just like a local 'Downloads' folder, except that these items are on a remote host.
Gmail FS: Your folders are the subject of your email and the files in the folder are the attachments.
Loopback FS: Mirrors some other user-specified directory, similar to creating a shortcut to another folder.

https://blog.box.com/box-drive-how-we-made-all-box-available-desktop

⇧ 「仮想ファイルシステム（Virtual File System）」なるものが出てきますと。

Wikipediaさんによると、

A virtual file system (VFS) or virtual filesystem switch is an abstract layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way. A VFS can, for example, be used to access local and network storage devices transparently without the client application noticing the difference. It can be used to bridge the differences in Windows, classic Mac OS/macOS and Unix filesystems, so that applications can access files on local file systems of those types without having to know what type of file system they are accessing.

https://en.wikipedia.org/wiki/Virtual_file_system

⇧ 様々なファイルシステムを抽象化した存在ということで、

https://www.researchgate.net/publication/254003497_Home_networking_as_a_distributed_file_system_view

⇧ 上記サイト様のイメージ図が分かりやすいかと。

「OS（Operation System）」によってファイルシステムが異なると思うのですが、「仮想ファイルシステム（Virtual File System）」が違いを吸収するような感じなんかな？

「仮想マシン（Virtual Machine）」のようなことを、ファイルシステムの領域で行っているような感じですかね。

Wikipediaさんの「List of file systems」の説明によると、

File system interfaces

These are not really file systems; they allow access to file systems from an operating system standpoint.

FUSE (file system in userspace, like LUFS but better maintained)
LUFS (Linux userland file system – seems to be abandoned in favour of FUSE)
PUFFS (Userspace filesystem for NetBSD, including a compatibility layer called librefuse for porting existing FUSE-based applications)
Secure Shell File System (SSHFS) – locally mount a remote directory on a server using only a secure shell login.
VFS Virtual Filesystem