[**Qubes OS**](../desktop.md#qubes-os) is an open-source operating system which uses the [Xen](https://en.wikipedia.org/wiki/Xen) hypervisor to provide strong security for desktop computing through isolated *qubes*, (which are Virtual Machines). You can assign each *qube* a level of trust based on its purpose. Qubes OS provides security by using isolation. It only permits actions on a per-case basis and therefore is the opposite of [badness enumeration](https://www.ranum.com/security/computer_security/editorials/dumb/).
Qubes uses [compartmentalization](https://www.qubes-os.org/intro/) to keep the system secure. Qubes are created from templates, the defaults being for Fedora, Debian and [Whonix](../desktop.md#whonix). Qubes OS also allows you to create once-use [disposable](https://www.qubes-os.org/doc/how-to-use-disposables/) *qubes*.
??? "The term *qubes* is gradually being updated to avoid referring to them as "virtual machines"."
Some of the information here and on the Qubes OS documentation may contain conflicting language as the "appVM" term is gradually being changed to "qube". Qubes are not entire virtual machines, but maintain similar functionalities to VMs.
Each qube has a [colored border](https://www.qubes-os.org/screenshots/) that can help you keep track of the domain in which it runs. You could, for example, use a specific color for your banking browser, while using a different color for a general untrusted browser.
Qubes OS is useful if your [threat model](../basics/threat-modeling.md) requires strong security and isolation, such as if you think you'll be opening untrusted files from untrusted sources. A typical reason for using Qubes OS is to open documents from unknown sources, but the idea is that if a single qube is compromised it won't affect the rest of the system.
Qubes OS utilizes [dom0](https://wiki.xenproject.org/wiki/Dom0) Xen VM for controlling other *qubes* on the host OS, all of which display individual application windows within dom0's desktop environment. There are many uses for this type of architecture. Here are some tasks you can perform. You can see just how much more secure these processes are made by incorporating multiple steps.
To copy and paste files and directories (folders) from one *qube* to another, you can use the option **Copy to Other AppVM...** or **Move to Other AppVM...**. The difference is that the **Move** option will delete the original file. Either option will protect your clipboard from being leaked to any other *qubes*. This is more secure than air-gapped file transfer. An air-gapped computer will still be forced to parse partitions or file systems. That is not required with the inter-qube copy system.
You can [copy and move files](https://www.qubes-os.org/doc/how-to-copy-and-move-files/) between *qubes*. When doing so the changes aren't immediately made and can be easily undone in case of an accident. When you run a *qube*, it does not have a persistent filesystem. You can create and delete files, but these changes are ephemeral.
The [qrexec framework](https://www.qubes-os.org/doc/qrexec/) is a core part of Qubes which allows communication between domains. It is built on top of the Xen library *vchan*, which facilitates [isolation through policies](https://www.qubes-os.org/news/2020/06/22/new-qrexec-policy-system/).
For additional information we encourage you to consult the extensive Qubes OS documentation pages located on the [Qubes OS Website](https://www.qubes-os.org/doc/). Offline copies can be downloaded from the Qubes OS [documentation repository](https://github.com/QubesOS/qubes-doc).
- [Arguably the world's most secure operating system](https://www.opentech.fund/news/qubes-os-arguably-the-worlds-most-secure-operating-system-motherboard/) (Open Technology Fund)
- [Software compartmentalization vs. physical separation](https://invisiblethingslab.com/resources/2014/Software_compartmentalization_vs_physical_separation.pdf) (J. Rutkowska)
- [Partitioning my digital life into security domains](https://blog.invisiblethings.org/2011/03/13/partitioning-my-digital-life-into.html) (J. Rutkowska)
