Everything you always wanted to know about Aspect ratio

4x3, 16x9, anamorphic lenses, 2.76:1. Here is a fantastic video discussing the various aspect ratios used in cinema, from the classic 1.33 to the extra large Ben Hur 2.76:1 (MGM65) without forgetting IMAX (1.43:1)

This video should explain everything you always wanted to know about Aspect Ratios but were afraid to ask.