Chris is right on the money with his explanations, so I'll just add this:
Because "modern" cinema theaters can only project in US widescreen standard (1:85:1) and scope anomorphic (2:40:1) aspect ratios, others such as the "Academy" ratio (a bit more than 1:33:1), 1:66:1, etc., cannot be projected.
When full frame-shot (Super35 or other 35mm spherical formats) movies are shown, a degree of image cropping will occur, although aperture plates are used to hide such things a frame lines and objects like microphones, booms and the like, while correctly (hopefully!) framing the active film image.
I personally don't mind the full frame format AR on video, mostly because it isn't a panned and scanned situation, and so long as the image doesn't suffer considerable dimensional losses, am a happy movie lover! http://www.avsforum.com/ubb/smile.gif