At login the C (and TC shell) reads initialisation files in your home directory. First it reads the file .cshrc (.tcshrc for TC shell) followed by the file .login.
In addition to environmental variables, both these shells have a number of local shell variables which are not exported to the environment.
When another shell is created to run a program, it only reads the .cshrc (.tcshrc) file, but it inherits any environment variables that were set up at login.