Zsh is a UNIX command interpreter (shell) usable as an interactive login shell and as a shell script command processor. Of the standard shells, zsh most closely resembles ksh but includes many enhancements. Zsh has command-line editing, built-in spelling correction, programmable command completion, shell functions (with autoloading), a history mechanism, and a host of other features.

This is the statically-compiled version of the shell, i.e. it doesn't require any dynamically loaded libraries to be present and will work even if libraries zsh (in general) needs are broken on the system. So it's more robust in case of a broken system.

It though also has a few limitations compared to the normal, dynamically linked, e.g. it can't use NSS related functionality which might result in minor imperfections like e.g. user names being shown in the prompt.