1. Medium-format film cameras took square 6x6cm pictures on 120 or 220 roll film, though some models took 6x7cm or 6x9cm pictures instead. The square aspect ratio does suit many subjects rather well, though it's often overlooked now. You can use Photoshop to crop any image to the square format, of course.
2. The most common aspect ratio today is the 4:3 ratio of compact digital camera sensors and Olympus and Panasonic's Four Thirds cameras. This is also the aspect ratio of conventional (non-widescreen) TVs, so it's ideal for playing back still pictures or movies on your TV at home.
3. Digital SLRs are different. They have a slightly wider 3:2 ratio. This may go unnoticed much of the time, but it makes a difference when ordering prints online. The standard 6"x4" snapshot size is a perfect match for the 3:2 images from a digital SLR, but not the 4:3 images from a compact. Here you need a print size closer to 6"x4.5" or 5.5"x4".
4. The 16:9 'widescreen' ratio offered on some cameras matches the aspect ratio of widescreen TVs, and it also produces very effective compositions with certain types of subject.
