Why some print sizes cut off parts of your picture
Aspect ratio is the numerical relationship between the height of the image and its width, and is usually expressed with two numbers.
All 35mm cameras use the same 24x36mm (3:2) format and those camera collectors out there--you know who you are--who want to argue about half-frame (18×24 mm) film cameras will have to admit that particular format never really caught on with the majority of photographers.
By comparison, digital SLRs and point-and-shoot cameras are available in many different aspect ratios and that can cause problems when making prints on standard-sized paper. A 4x6 print has an aspect ratio of 3:2 making it perfect for some cameras (no cropping) while an 8x10 has an aspect ratio of 4:5, making it "ideal" for others.
There's a bit more to the story than just mismatched aspect ratios. Digital camera sensors are often cropped internally to match up with one of the standard print aspect ratios, and that can cause some problems in the "what you see is what you get" department.