The Kuleshov effect is one of the most famous cinema principles. Russian cinematographer Lev Kuleshov found in the twenties that two shots processed one after the other are processed together in our brains.
He filmed a small film where he showed a pretty much inexpressive shot of an actor (Ivan Mozhukhin) along other images, like a small child, a corpse and food. Even though his face remains the same, the audience 'sees' different expressions in the same face: sorrow, desire or hunger. It was later used to the full in many abstract films, like Buñuel's Un Chien Andalou.
The Kuleshov effect is perhaps one of the most graphical demonstrations of the power of montage and editing in films, an area often overlooked by most people. There is an excellent short documentary made by Mark Rappaport in 1995 with Mary Beth Hurt that shows the Mozhukhin clips, and further commentary.
Further reading and reference:
http://mason-west.com/Film/clips.shtml
http://www.ouc.bc.ca/fina/glossary/k_list/kuleshov.html
http://www.arts.uwaterloo.ca/FINE/juhde/ju-952.htm
Thanks to SharQ for his additions.