a) Introducing the shell
There is usually a program in between the user and the computer called a command shell. What the user types goes into the shell, which then figures out what commands to run and orders the computer to execute them. (Note that the shell is called “the shell” because it encloses the operating system in order to hide some of its complexity and make it simpler to interact with.)
http://swcarpentry.github.io/shell-novice/01-intro/