Every process except process 0 is created when another process executes the fork system call. The process that invoked fork is the parent process, and the newly created process is the child process. Every process has one parent process, but can have many child processes.
The kernel identifies each process by its process identifier (PID). Process 0 is a special process that is created when the system boots; after forking a child process (process 1), process 0 becomes the swapper process. Process 1, known as init, is the ancestor of every other process in the system and enjoys a special relationship with them.