Explanation:
A shell is a command-line interface (CLI) program that allows users to interact with the operating system by entering commands. It acts as an intermediary between the user and the OS kernel.

Common tasks performed using a shell include file manipulation, program execution, and system configuration. Popular shell programs include Bash (Bourne Again Shell), Zsh, and Windows PowerShell.

The shell parses user commands, translates them into system calls, and executes them with the help of the operating system. It can also run shell scripts—text files containing a series of commands.

Formal Definition:
A shell is a command-line interpreter that allows users to interact with the operating system by entering commands, facilitating resource management and task automation.

Additional Information:
There are two types of shells: command-line shells (e.g., Bash) and graphical shells (which provide a GUI for users to interact with the OS).

Advanced shell features include command piping, environment variable management, and command substitution. Shells are integral to system administration, programming, and automation tasks.