Plan 9 is an operating system kernel but also a collection of accompanying software. 
The bulk of the software is predominantly new, written for Plan 9 rather than ported from Unix or other systems. 
The window system, compilers, file server, and network services are all freshly written for Plan 9. 
Although classic Unix programs like dc(1), ed(1), and even troff(1) have been brought along, they are often in an updated form. 
For example, troff accepts Unicode documents encoded in UTF-8, as does the rest of the system.

Plan 9 is a distributed operating system, designed to make a network of heterogeneous and geographically separated computers function as a single system. 
In a typical Plan 9 installation, users work at terminals running the window system rio, and they access CPU servers which handle computation-intensive processes.