The kernel doesn't support threads per se, there's only one process
type.  Processes may share memory.

Libthread is a user level library that allows multiple threads
per process, in addition to multiple processes.  It also
provides typed CSP like channels.

As a user of libthread, you can start a thread in the current process
	threadcreate()
or in a new process
	procreate()
Threads created in the same process use cooperative scheduling,
i.e, when one yields the processor (implicitly by calling one
of the channel functions or explicitly by calling rendezvous)
another can run.  However these threads are only logicly separate.
Since the kernel knows nothing about them, they can only run
sequentially on a single CPU.  If one calls a blocking system
call (read, sleep, etc) none of the others will run.

Therefore, a process that wants to make use of multiple processors
and/or that wants to allow some threads to block on system calls
will use proccreate (which in turn calls rfork) to create threads in
multipe processes.  You can mix and match; the threads look the same
as far as communication is concerned.