Whilst the operating system can run many processes at the same time, in fact it only ever directly starts one process called the init (short for initial) process. This isn't a particularly special process except that its PID is always 0 and it will always be running.

All other processes can be considered children of this initial process. Processes have a family tree just like any other; each process has a parent and can have many siblings, which are processes created¹ by the same parent.

Certainly children can create more children and so on and so forth.

init-+-apmd
     |-atd
     |-cron
     ...
     |-dhclient
     |-firefox-bin-+-firefox-bin---2*[firefox-bin]
     |             |-java_vm---java_vm---13*[java_vm]
     |             `-swf_play