In summary: Urbit is a functional/deterministic VM. That's mostly it. It runs a bytecode called Nock. The only language that really compiles to Nock is a weird language called Hoon. Because the VM is functional and deterministic, it is (in theory) also portable – you pick up the (ever-evolving) VM image and move it somewhere else, and it should act the same. Since the VM may move around there's also an identity/networking layer so you can talk to the VM wherever it is.
Right now Urbit is a program called u3 (runs on Linux or whatever) that runs Nock programs. OS 1 is such a program that does a couple things. It's unclear to me if they've really built an OS or just a library of routines that can be used to build a single multipurpose application.
The most direct article I found was "Review of Urbit": https://medium.com/@noahruderman/review-of-urbit-e7cc4c35f14...
Urbit's own "Common Objections to Urbit" was helpful, in a methinks-he-doth-protest-too-much sort of way: https://urbit.org/blog/common-objections-to-urbit/
And that article pointed me to "Houyhnhnms vs Martians" which was helpful though also as weird as Urbit: http://ngnghm.github.io/blog/2016/06/11/chapter-10-houyhnhnm...
While some people say Urbit is feudalistic, I tend to agree more with those that call it simply obscurantist. More occult than fascist.
In all seriousness, it's nice to see that the Urbit peoplo are actually making substantial progress on... whatever it is they're doing. I love huge, totally outlandish projects like this.
hierarchical
p2p network
of virtual machines
running an unusual, but lisp-like, OS
where the OS state is a pure function of a stream of input events
That's about what I got from reading through the webpage.
(As an aside, the whole "$PROVIDER is going to hack into my VM" thing is pretty overblown. The cloud has existed for over 10 years and the worst that happens is the provider deletes all your data and you restore from backup onto a different provider. You do have a backup that isn't on the same provider, right?)
The worry is not that your provider will hack you, the worry is that your provider will silently steal your trade secrets.
Cryptographic keys that control identity and money are good distilled examples of this.
That's the "what" it's just that the "how" (fucking hoon/nock) and "why" (moldbugs weird shit) is such a fucking rabbit hole that it obfuscates the very straight forward and uninteresting "what".