If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating a script.
From the Python documentation. I’m enteratined by the implication that the primary way to use Python is at the interactive console, but it also happens to have a obscure feature where you can run a program saved in a file.