tcsh is an enhanced but completely compatible version of the Berkeley UNIX C shell, csh.
Quoted from the manpage:
tcsh is an enhanced but completely compatible version of the Berkeley UNIX C shell csh It is a command language interpreter usable both as an interactive login shell and a shell script command processor. It includes a command-line editor, programmable word completion, spelling correction, a history mechanism, job control and a C-like syntax.
tcsh is the default shell on FreeBSD & DragonFly BSD, and was the default shell on earlier versions on Mac OS X.
Since tcsh is completely compatible with csh, almost all versions of csh encountered are actually tcsh.
Official site: http://www.tcsh.org/