A problem some people experience is that parts of their digital photos get cut when printed. This is due to the difference between the aspect ratio of your digital photo and the paper size of the print.
The aspect ratio is the horizonal dimension related to the vertical dimension. With a paper print, these are the inches/centimeters of the print - so a 6" x 4" (15 cm x 10 cm) print would have a ratio of 1.5:1 (or 3:2 in whole numbers). A 7" x 5" print would have a ratio of 1.4:1.
With a digital photo the aspect ratio is calculated using its pixel dimensions. A camera taking a 2048 x 1536 pixel image has an aspect ratio of 1.33:1. One with 3504 x 2336 has a ratio of 1.5:1 (which not coincidently is the ratio of 35mm film). If an image with a ratio of 1.33:1 is printed on 6" x 4" paper (1.5:1), to fill the 6" x 4" paper the printer can either add whitespace around the photo, or crop (cut off parts) of the photo to make it fit. Most printing services do the latter
Your original digital photos, whether they be from your digital camera or from a scanner are your "negatives" and should never be touched. Keep them in a safe place and always work on copies of your originals. See the Storage Page for details about how to properly save and archive your originals.