Commander is a Ruby library that provides tools for building command line interfaces. It allows defining commands, options, and receiving input. Some key features include: - Defining commands with descriptions, syntax, and actions - Support for command options like prefixes, suffixes, and other parameters - Interactive input using Highline for questions, passwords, choices - Customizable with features like progress bars, speech synthesis, and table output The library provides an easy way to build full-featured command line applications and interfaces in Ruby.