User: Since Un*x is a multiuser system, any process needs a User ID that will be used by the kernel to limit the privileges of the executed command.
Process ID: Every process has a PID that allows the user to uniquely identify the process in a running system. The PID is usually the process position in the kernel process table.
Parent Process ID: every process records the Parent Process ID,
so he knows who is his father. When a process exits the kernel
sends a SIGCHLD signal to the Parent process, if the parent does not
handle this signal the child stays in <zombie> state of (defunct)
until his parent exits or it handles the pending signal.
If the father of a process exits before his child then
the kernel sets the PPID to 1.
Process 1 is the Init process, it is started by the kernel; if this process gets killed, then the system shuts-down.