Zircon is the core platform that powers Fuchsia. Zircon is composed of a microkernel (source in /zircon/kernel) as well as a small set of userspace services, drivers, and libraries (source in /zircon/system/) necessary for the system to boot, talk to hardware, load userspace processes and run them, etc. Fuchsia builds a much larger OS on top of this foundation.
The canonical Zircon repository is part of the Fuchsia project at: https://fuchsia.googlesource.com/fuchsia/+/HEAD/zircon/
The Zircon Kernel provides syscalls to manage processes, threads, virtual memory, inter-process communication, waiting on object state changes, and locking (via futexes).
Currently there are some temporary syscalls that have been used for early bringup work, which will be going away in the future as the long term syscall API and ABI surface is finalized. The expectation is that there will be about 100 syscalls.
Zircon syscalls are generally non-blocking. The wait_one, wait_many
port_wait and thread sleep being the notable exceptions.
This page is a non-comprehensive index of the zircon documentation.