Process
Every process in Linux has an ID, so-called PID.
Whenever we create a new process via fork() (or clone()) system call, the kernel assigns a next spare PID to it.
The process that makes the fork() call becomes a parent of the newly created process and its PID becomes a parent process id, i.e. PPID of the child process.
This procedure is respected for all the processes in the system, starting from the very first one (i.e. init process with PID 1).
Thus, processes in Linux form a tree-like structure where at any given time, a process always has a single parent process (ancestor) and can have from 0 to N child processes (descendants):