Linux Filesystem Hierarchy

pipes and sockets
Special files that programs use to communicate with one another. They are rarely seen, but you might
be able to see a socket or two in the /dev/ directory.
process identifier
Shown in the heading of the ps command as PID. The unique number assigned to every process
running in the system.
rpc
Remote Procedure Calls. It enables a system to make calls to programs such as NFS across the
network transparently, enabling each system to interpret the calls as if they were local. In this case, it
would make exported filesystems appear as thought they were local.