Zsh, or the Z shell, is a Unix shell that can be used as both an interactive login shell and a command interpreter for shell scripting. It's an extended version of the Bourne Shell (sh) with a large number of improvements, including some features from Bash, ksh, and tcsh.
Zsh has features such as automated suggestions, spell correction, a rich programming environment, and 'theming' capabilities, making it a favorite shell for many developers. It also provides tools for manipulating and querying the command history, and powerful scripting capabilities.
Overall, Zsh is well known for its high degree of configuration and its numerous features that improve user efficiency.