A shell is a command-line interface that allows users to interact with an operating system (OS) by entering commands. It is a program that interprets user input and executes the corresponding commands. The shell provides access to the OS's services and can execute other programs, manage files and directories, and control input and output. The most common shells in Unix-based systems are Bash, Csh, and Zsh, while Windows uses the Command Prompt and PowerShell.