When the operating system is started after a boot, a single process is started. This process is the parent of all subsequent processes. Each process created on the system has a unique number known as its PID associated with it.
When you login to the system a process is started to run your shell program. Any processes that are started from within your shell - such as entering a command - are the children of this process. A process can have many children, but only one parent.
See Managing jobs and processes for further information.