Shell commands are instructions used in a command-line interface to interact with the operating system. They allow users to perform various tasks, such as managing files, running programs, and configuring system settings. Common examples include commands like ls for listing files and cd for changing directories. These commands are executed in a shell, which is a program that interprets user input and communicates with the operating system. Different types of shells, such as Bash and Zsh, offer unique features and functionalities, making it easier for users to automate tasks and streamline their workflows.