When people think of prints they usually think of them in the traditional print sizes, such as 4×6, 5×7 and 8×10. When they place their print order they are usually confused as to why their images vary from size to size.
The camera takes a photo that when uncropped with reproduce to a 2″ x 3″ ratio, which translates to a 4×6, a 6×9, an 8×12, a 12×18, a 16×24, a 20×30 and so on. The standard print (and frame) sizes are 4×6, 5×7, 8×10, 11×14 and 16×20. So, when you deviate from that crop ration you start to lose parts of your image. Sometimes you lose significant amounts of your image!
Here is an example for you to see how much of the image you lose when you purchase different sizes. Look at how much that 8×10 image lost! An 8×10 print actually crops off two inches from the original. Two inches is a lot when it comes to print sizes!
These are things to keep in mind when you are looking to purchase prints or if you have a CD and are going to print the images yourself. Each image will vary slightly based on the crop size. If you are using a CD to print your images, you should also remember that sometimes it is not a person who sizes your image so they may not even see it. Their computer or photo system may just load up the image and crop it. You should always print the images where you can manually move them around and crop them the way you want. Photographers generally crop images the way they think is best, but if you have a preference you should definitely convey that to your photographer!
I hope that helps clear up any confusion about print sizes for you!