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.