Command line arguments (i.e. arguments that are passed to the program via the command line) are used to supply information to a program when it first starts running. For example, we might want to tell the program the name of a file to open and process. Or we might want to pass different "options" to the program (e.g. "-v" is traditionally passed to a program when you want it to be "verbose". That is, you want the program to print out as much information as possible so you can debug).