The shell is a command interpreter and it is your interface to the UNIX operating system. When you log in to the system, a shell is started to receive your commands. There are many shells to choose from, all with different features and enhancements. The default shell is the Enhanced C-Shell (tcsh).
csh is an enhanced version of the Bourne shell, providing many new features such as aliasing, command history, and job control, to name just a few.
tcsh is the same as the csh, except it provides command-line recall and editing based on Gnu EMACS editing commands. tcsh provides many more enhancements than just command-line editing; a discussion of the options can be found in the online manual pages for tcsh.