When executing a program on UNIX, the system creates an environment for that program. In this setting there is everything needed to execute that program. The OS tracks processes though a five digit number Recognized as the Process ID. Each process has its own unique PID. PID’s ultimately repeat because all the probable numbers are used, and the next PID repeats the system again. At any time, no two Processes with the same PID occur in the system, because it is the PID that UNIX uses to track its processes.
Each UNIX process has two ID numbers. The PID or process ID, and PPID or Parent process ID. Each user process in the system has a PPID. Normally, when a child process is killed, the parent process is told by a SIGCHLD signal. Then the parent can do another task, or restart a new process as needed. When a process is killed, a process listing may still show the process with Z stats. This is a zombie, or obsolete process. The process is dead and not being used. These processes have finished the execution but still have an entry in the process table.
Daemons are system related background