Qubes OS is an open-source Linux Desktop operating system designed for security. Its main premise is that you can always be compromised and the only real solution is isolation.
To achieve this, the Operating System consists of multiple virtual machines under Xen, called Qubes, each of them completely isolated one from another.
The Qubes are based on secured templates, which are read-only. Only your personal data is persistent, which does help eliminate possible backdoors in case of a compromise.
Qubes also offers Disposable VMs where you can browse untrusted websites or run untrusted applications. Once they are closed, everything related to the VM is removed. This is especially useful as you can open any file, be it a PDF or a JPEG, in a disposable VM, therefore preventing any exploits or malware infecting your PC.
It should be noted that Qubes OS is designed as a single-user system and as the single operating system running on your computer. You are not able to create multiple accounts and share the computer with someone else.
Qubes OS is not easy to use nor easy to configure. It’s made for power users who require a high grade of security. Getting used to it might take some time, but if you really care about your security, there is no other choice at this moment.
For more information about Qubes OS, please check their official website – https://www.qubes-os.org/