What’s what
Most users of personal computers today are intimately familiar with the graphical user interface (GUI) whereby interactions with the computer consist of using a mouse (and occasionally the keyboard) to navigate across several aethetic applications, often clicking on visual elements such as folder icons, buttons, and menus.
Before the development of the GUI and means of interacting with the computer through these more-visual elements, there was (and continues to be) a text-based method of interacting with the computer called a command-line interface (CLI).
In the “old days,” a computer would boot into the CLI, as there was no other option. Today, each major operating system has an application that provides a CLI. This type application is called a terminal emulator (or just terminal for short). On MacOS, the app is unceremoniously named “Terminal” and can be found in the /Applications/Utilities directory. But you can download other terminal applications, including iTerm2, Warp, Alacritty, Hyper, Tabby, Kitty, Rio, or others.
Every terminal application has a text-based input field called the command prompt. When you type and submit a command at the command prompt, the Shell Command Line Intepretor (or just Shell) parses and interprets the text you have typed, carrying out your instructions and possibly returning information to you in the terminal. This Read-Eval-Print loop (REPL) is common when computer programs allow “live” interaction with between the user and computer. The default shell on MacOS at the time of writing is zsh (pronounced the “Z shell”) which replaced bash (the “Bourne Again SHell”) as the MacOS default starting with the release of MacOS Catalina in 2019. bash, as evidenced by its name, was an evolution of the Bourne shell (with the short name sh). To round out the list, the only other popular shell today is fish (the “friendly” shell).