Tools·intermediate
Shell
The program inside the terminal that interprets your commands. Usually bash or zsh.
When you type cd Desktop into a terminal, something reads those characters and figures out what to do. That something is the shell.
The terminal is the window. The shell is what runs inside it. Modern Macs default to zsh. Older Macs and most Linux systems use bash. Windows has PowerShell and Command Prompt.
Mostly compatible for everyday commands — cd, ls, mkdir, rm. Scripts get shell-specific fast. You don't need to worry about the difference until you start writing shell scripts.