When logging into a UNIX/Linux box, which profile scripts are executed?

When logging into a UNIX/Linux box, which profile scripts are executed? Typically there are several different profile scripts that set the $PATH variable and other aspects of the user's environment. However in most user's I've seen, they have .cshrc for CSH, .bashrc for bash, kshrc for ksh, etc.. Do they all run upon login? Is it based on the user's default shell? How does the login script mechanism work? For example when using su - which script would run?