Processes and jobs

Every running program is a process. A process has a number, the PID, which the system assigns at launch and does not reuse while the process is alive. It also has a PPID, the number of its parent, the process that started it. That builds a tree: your shell spawned a command, the command spawned another.

Processes are born and die all the time. To manage them you first have to see them, then send them a signal, then split the work between the foreground and the background.

ps gives a snapshot of every process:

$ ps aux                 # all processes, all users, in detail
$ ps aux | grep nginx    # filter by name

In the ps aux output, the PID and PPID columns say who spawned whom, %CPU