In the film days photo paper sizes were designed to fit the aspect ratio of the film negative frames. Digital camera sensors use different aspect ratios and as such when printing digital photos on traditional paper the photos have to be either cropped or shrunk.
There are many ways in which you can print your digital photos. You can use paid digital photo printing services, you can print at home or you can find free prints offers on the Internet. Regardless of the method you will end up choosing the paper size on which to print the digital photos. If you pick a traditional photo paper size like 4 inches by 6 inches or 5 inches by 7 inches you will also have to choose if you wish your digital photos to be cropped or shrunk.
The reason is different aspect ratios. Old film cameras used negative frames with an aspect ratio of 2:3 or in other words if you divided the height of the sensor by its length you would get 2:3. The most common tradition photo paper size was 4 inches by 6 inches which is exactly 2:3 aspect ratio. In other words printing a film photo on a 4 by 6 resulted in a perfect fit. New digital camera sensors however have an aspect ratio of 3:4 which means that trying to print a digital photo on a 4 inches by 6 inches photo paper will result in the photo not fitting. The photo needs to be cropped or shrunk in order to fill the whole paper or alternatively you can print the whole photo and leave a white empty strip.
With digital cameras becoming more and more popular photo printing services and providers of free prints too introduced new photo paper sizes that were designed for digital cameras. The new sizes are known as 4 by D, 5 by D and so on. The D stands for the fact that they are Digital photo safe. A 4 by D photo paper is 4 inches high by 5.33 inches long. If you divide the two you get the exact digital camera sensor aspect ratio of 3:4
Many photo printing services today offer both traditional and digital safe photo paper. One way of thinking about digital safe photo paper is that instead of leaving an empty white strip when printing digital photos that white strip was just cut and the photo paper was made smaller.
Since many photo prints end up in photo albums the market has also seen the emergence of digital prints safe photo albums. Such albums have pockets that are designed to perfectly fit the slightly different size digital safe photo paper. Over time the traditional photo paper sizes will phase out and eventually photo printing services as well as free prints services will end up offering only digital photo safe prints. Until that point in time you should pay attention to choosing the correct paper size when printing your photos. Usually the photo printing service will warn you if there is no fit between your digital photos and the paper size you chose.
Danette Mckay explains about this subject in more depth at photo printing