A set of links devoted to the one of the greatest (if not simply THE greatest as is the popular opinion), most complex (plot- and technique-wise) and easily pathbreaking films ever made:

*A Viewer's Companion to 'Citizen Kane' which explains among others, Deep Focus, Visible Ceilings and Matte Drawings

* The CITIZEN KANE Review by ROGER EBERT (spoilers present: watch it first!)

While you're at it, you might want to walk through Ebert's reviews of other great movies including The Apu Trilogy.