Urbit's userspace is managed by Arvo's Gall vane.
An overview of userspace in Urbit.
Resources on Gall which manages userspace applications.
Resources on the flagship web client Landcape, through which most users interact with their ship.
Resources on threads - monadic functions useful for complex IO.
Resources on Graph Store, a non-relational database specialized for social media applications.
Resources on distribution and management of desks/apps.