When you login to the system, your initial working directory is set to your home directory, and selected configuration files often called ``dot files'' are read by the system to set up your work environment. (Files with names that begin with a period are normally not displayed by ls, and therefore configuration files usually have names that begin with a ``dot''.
The .cshrc file runs first, and also whenever you start a new shell. This file does most of the work in setting up your environment. The locations where tcsh looks for programs that can be run are designated by the PATH environment variable, or the path shell variable. Similarly, the locations of man pages are set with the MANPATH variable. PAGER is used to select the program that allows you to scroll through a long document a screen at a time, and is usually less or more. EDITOR and VISUAL are used to select which editor you would prefer to use by default in certain utilities. PRINTER changes the default printer lpr uses.
You can also set various other tcsh options in your .cshrc file, or change the default access permissions a newly created file is assigned. Look through the default .cshrc file and the csh(1) and tcsh(1) man pages for more information and examples.
Your .login file normally runs after .cshrc, and only when you first login to a system (normally not whenever a new shell is started). It is primarily used at the IMA to print the Message of the Day (usually the day's schedule). Your .logout file likewise runs when you log out of the system.
The .xsession file runs whenever X is started from xdm, when you first login. It starts up a few X clients (the clock, load meter, xbiff, and a couple of xterm or rxvt windows), and your window manager. If you want a certain X client program to always start up when you login in the morning, it should probably be started by this file.