You probably have come across the issue of aspect ratios before, even if you were unfamiliar with the effect and what was happening. Aspect ratio is the relationship between the width and the height of an image. With having different sizes of digital cameras these days, it's much more common than it was with 35mm film to notice this problem. You will see cropping happening if you print your photos, whether at home or by a lab. What happens is that standard print sizes have stayed the same, but they don't always match the dimensions of your digital images.
The most common ratios in digital cameras are 3:2, 4:3, and 5:4. There is however other ones such as 16:9 (modern HD TVs have this ratio), and 1:1 (square image). The first number represents the width of an image, and the second number (after the colon) represents the height. These numbers are not specific measurements, but the relationship between the two.
Some digital cameras, along with 35mm film, have an aspect ratio of  3:2 (also sometimes referred to 1:1.5 ). This proportion will make a 4x6 print without any cropping. This ratio is often referred to as the "golden rectangle" since most people prefer how this ratio frames their images. Other digital cameras may have a ratio of 5:4 which will produce an 8x10 image without cropping, but if printed at 4x6 or 5x7 will incur some cropped areas of the original image. The newer 4/3 cameras on the market (such as Olympus) produce a... 4:3 ratio!
To figure out the ratios of print sizes, just flip the numbers so that the width is the first dimension, and then reduce the numbers down to their lowest value. So, 4x6= 3:2, 5x7= 7:5, 8x10= 5:4.
Another thing to keep in mind is that if you have a 3:2 image for example, the larger the print, the more of the image you are loosing with the crop. A 3:2 image will produce a 4x6", 5x7.5", and a 8x12" image. So a 3:2 image printed at 4x6 will loose nothing. A 3:2 in a 5x7 will loose a small amount on the sides (a 1/2 inch). A 3:2 printed to an 8x10 will loose four times what you lost on the sides of the 5x7 print (2 inches).

Some of the Panasonic point and shoot digital cameras actually let you choose which ratio you want to shoot in (above). But this is not a standard choice for most cameras, whether is be a point and shoot or SLR.

So, what can you do to combat this ratio and cropping problem? There's a few solutions.
* You can loosely frame your images during the shooting process so that no matter what size you decide to print, you won't be loosing any important information in your crop.
* You can determine your favorite print size and purchase a camera that produces an image with that ratio, or vise versa, only print specific print sizes that match the ratio of your cameras image.

If you are printing your images yourself, then you have the ability to make the crop selection yourself. If you send your images out to be printed however, you should remember to crop to the print size you want before sending your images off to ensure that the crop goes where you want it to go. To make sure you have a proper ratio crop, you will need to have photo editing software that either allows you to input a crop ratio, or where you can turn on the rulers and a grid to follow while you digitally crop the image.

While you, the photographer, may now understand crop ratios, this is a much harder thing to explain to clients who want to order prints. Something I find genius is the way that myCapture shows the crop selection options during their print ordering process. myCapture is the service that many newspapers are using on their sites for readers to purchase photos that they have seen in the newspaper. As you can see above, they show which areas will be cropped out with which cropping selection. In addition, they offer a white border option so that the image isn't cropped at all... but with this you will see a white border if your 5x7 print is framed with a 5x7 frame. The times when you may want to choose a white border option is if you will be putting the photos in an album, a free floating type frame, or having a custom mat made for your image and frame. In these cases, a standard print size isn't usually as necessary.