(content here copy-pasted from this HN thread, because I thought it was a good summary)
What is Urbit?
Urbit is a virtual machine OS for server-side applications. If you imagine a future in which it is common for non-technical people to rent cloud server space on which to host server-side applications (say, a small blog, a mastodon node, a minecraft server, etc), Urbit aspires to be a good platform on which to host them.
- All input events (http request to an urbit-based api, signed message from another urbit, keystroke from console, etc) are transactions which change the OS state (or don’t, if they fail). As a result, it should be impossible for a transaction to fail halfway through and leave the urbit instance hosed.
- Exactly-once messaging between nodes. This is possible because nodes have persistent connections; disconnection is indistinguishable from long latency. This may sound minor, but it is a huge part of what makes urbit novel and (theoretically) stable and secure.
- Built-in identity and auth. An urbit instance can’t boot without an identity, which serves as username, network-routing address, and also as the public key with which all outgoing messages are encrypted. In practical terms, this means no urbit app or service needs to deal with logins or passwords or crypto.
- There are only 2^32 first-class identities, which makes urbit a de facto reputation network. This is to minimize malicious behavior; if an identity costs $5, and you can only make $2 from spamming before that identity is blacklisted, no one will spam.
- The urbit network is hierachically federated, and hence resistant to censorship. (Of course, this is a misfeature if you want to be able to censor people off of the networks you participate in)
- It’s not there yet, but the urbit kernel aspires to be so small and simple and formally-provably-correct that at some point it’s done. As in, done done – no features to add, no bugs to fix, done. A lot of the design decisions (some of which are wildly unperformant) make no sense unless you take this goal in to account. More on that here: https://urbit.org/blog/toward-a-frozen-operating-system