C3: C in Urbit
Under u3 is the simple c3 layer, which is just how we write C in Urbit.
U3: Land of Nouns
The division between c3 and u3 is that you could theoretically imagine using c3 as just a generic C environment. Anything to do with nouns is in u3.
U3: API Overview
A walkthrough of each of the u3 modules.
Cryptography
References on the cryptography libraries utilized by jets.