When you enter a command it invokes a program. While this
program is running it is called a process. It is important to
grasp that although there is only one copy of a program held in
the file system, any number of processes can be invoked which
run this program. 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.