Spotlight: Choosing between "C'est" and "Il est"

This page of the grammar section deals withchoosing between c'est and il est. On the surface, either can potentially be used to translate "it is". But which is used when?

http://www.french-linguistics.co.uk/grammar/c_est_il_est.shtml

As with all of the grammar pages, comments are welcome in the Comments section at the bottom of the page.