What if that's not a command?

What happens if you type a command your computer doesn’t know?

It looks to see if it can find a program to run with that name. But it doesn’t look everywhere: it has a list of all the directories (or folders) to check. It goes through that list, checking each place. If it finds a program with that name, it runs it. Otherwise, it looks in the next directory on its list.

If it gets to the end of the list and hasn’t found a match, it gives up: there is no command or program to run. It will tell you why it didn’t obey the command you gave it (the actual words depend on which command line interpreter you are running), but will usually look something like this:

banana: command not found

or

The term 'banana' is not recognized as
the name of a cmdlet, function, script,
file, or operable program.

On most systems, you can use tab completion to know before you press enter if the command exists.