The first thing to recognise here is that most digital cameras take pictures at a different ratio to traditional film cameras. Digital cameras were orginally designed to capture images for computer monitors, and as you probably know, these have dimensions typically like 640 x 480 and 800 x 600. If you divide the height by the width, you will find that the ratio is usually 4:3.
A traditional camera takes pictures that fit perfectly into a 6x4" photo. If you again divide the height by the width, you find that this is a different ratio, 3:2. So whats the problem you ask? Nothing really, but you have a decision to make when you come to print your photos. The standard photo sizes, and frames were designed for film and the 3:2 ratio. See our article on cropping for a visual description of the issue.
What has confused the issue more recently is that high end cameras including DSLR's now default to a 3:2 ratio for shooting but also offer 4:3. You can check your quality setting to see what ratio you are taking photos in, or simply do the pixel division. Take the small number and divide by the large number. If you get .75 then you are shooting at 4:3 if you get .66 you are shooting at 3:2.
The table below details which commonly available print sizes are best for your images. If you have a digital camera, most probably you will need to choose a print size from the first two columns.