A job is a process launched from the shell by a person. A job receives a PID because it is a process and it receives a job number because it is a job.
In normal operations when a parent process terminates (like the terminal window) then all of its child processes must terminate.
A special question is: how do you launch a process (program) in the shell and keep it running after you close the terminal window, and even after you log off of the distro?