What is Zsh?
Zsh is a UNIX command interpreter (shell) which of the standard shells most resembles the Korn shell (ksh); its compatibility with the 1988 Korn shell has been gradually increasing. It includes enhancements of many types, notably in the command-line editor, options for customising its behaviour, filename globbing, features to make C-shell (csh) users feel more at home and extra features drawn from tcsh (another `custom' shell).
Comments and Reviews
Said about Zsh as an alternative
Compared to BASH, Zsh has support for many more plugins and addons of the sort. There exists a framework for this very purpose called "Oh-My-Zsh" and it can be used to add things like themes and support for command auto-complete. Zsh is pretty much just BASH but with more features built on top.