In addition to sqweek's good reply:
The "distributed" part also refers to how a typical installation is
structured. The system responsible for authenticating you, your
file server, the cpu server you run processes on, and the terminal
you're typing at may well all be distinct computers, but for the
most part none of the application code knows anything about
networking. The system takes care of it for you (mostly thanks to
9p, as sqweek described).
In terms of concurrency, that's more of a programming question
than an OS question (which isn't to say the OS isn't relevant).
Plan 9's thread(2) library is probably the most relevant thing
there. It follows a very different (and easier to learn, read, and
write) model than the threads you see in other systems. Plan
9 mostly helps there by making things lighter, encouraging the
use of multiple processes where other systems penalize it; the
library itself is now available on unix through plan9port.
Anthony
