On the Canon 60D it is possible to set the aspect ratio's from 3:2 - 4:3 - 16:9 - 1:1. The manual explains how to set it but Why and when is not explained. Is there a quality or size difference between them?
The ratios start with your camera sensor (3:2), and vary for your end product. The 3:2 and 1:1 ratios are only in the stills category; 3:2 using the full sensor area (to crop later in "post"), and 1:1 is pre-cropped for square photos (like user's photos here on Creative Cow). Movie recording only comes in 16:9; both 1080p (at 2Megapixels)and 720p (at 1Megapixel), and 4:3 with 480p (at .3Megapixel). 16:9 is HD (digital TV,DVDs,web HD), 4:3 is SD (original TV format,web SD). You want the most sensor area for quality (stills or movie), but the end user's display is the real word "crop factor". When in doubt, shoot highest quality your storage will allow, change format/aspect ratio in initial or export transcode (i.e. compressor/mpeg streamclip). Hope this helps...ML