We use "digital sizes" to describe prints with an "aspect ratio" of 3:4 as this is the most common size for digital cameras, and "conventional sizes" to describe prints with an aspect ratio of 2:3 as this is what 35mm film users are used to. "Traditional sizes" of 5"x7", 8"x10" and 10"x12" do not match either of these aspect ratios but are still popular and therefore offered by us.
Aspect ratio describes the height-to-length relationship of an image. A square has an aspect ratio of 1:1 as the height always matches the length – for every one unit of height there is an identical unit of length. The aspect ratio of a rectangle is arrived at by comparing the length to the height. With a 4"x6" print, this rectangle has two units of height for every three units of length, and hence has an aspect ratio of 2:3
The easiest way to visualise this it is to think of televisions. We were all used to the traditional TV shape (which has an aspect ratio of 3:4). Then longer widescreen televisions came along. Now when we watch old programmes or videos, we sometimes see a border on the left and right side of the screen as the aspect ratio of the broadcast is shorter than the aspect ratio of the widescreen TV.
The majority of digital cameras capture images in an aspect ratio of 3:4, though digital SLRs use an aspect ratio of 2:3 and some cameras can be switched between the two. If you want to avoid cropping (which can result in heads being cut off, for example) you need to pick a print with an aspect ratio which matches the image.
If you are not afraid of maths, you can calculate the aspect ratio of your images. Look in your camera's manual for its resolution (e.g. 2048x1536). Divide the larger number by the smaller one. If the result is 1.33 (e.g. 2048 divided by 1536) then you need "digital" size prints such as 4"x5.3" (as 5.3 divided by 4 equals 1.33). If the result is 1.5 (e.g. 3000 divided by 2000), this requires a "conventional" size print such as 4"x6" (as 6 divided by 4 equals 1.5).
Don't worry if you find this complicated – just pick digital sizes and see what comes back in the post.