(csh) The Unix command-line interpreter shell and script language by William Joy, originating from Berkeley Unix.
Unix systems up to around Unix Version 7 only had one shell - the Bourne shell, sh. Csh had better interactive features, notably command input history, allowing earlier commands to be recalled and edited (though it was still not as good as the VMS equivalent of the time).
Presumably, csh's C-like syntax was intended to endear it to programmers but sadly it lacks some sh features which are useful for writing shell scripts so you need to know two different syntaxes for every shell construct.
A plethora of different shells followed csh, e.g. tcsh, ksh, bash, rc, but sh and csh are the only ones which are provided with most versions of Unix.