As a photographer who likes to get in close and fill the frame with the object, I have to be very careful that I leave enough space around the object for printing. This is called the printed aspect ratio. My camera has a ratio of 2 X 3 which means my 4 X 6 photographs will print exactly as I take them.
When we travel in our 1940 Ford pickup we pull a small trailer for our luggage, lawn chairs, snacks and ice chest. When I took this photo, I wanted to fill the frame with the truck so the trailer would not show. If I were to have this printed into a 4 X 6 print, this cropping looks fine.
What would happen if I want a different print size. If I've not allowed enough extra space, I'll be disappointed in the print. For instance what if I want a 5 X 7. It's too close to the front and back bumper.
Now let's say, I really want an 8 X 10. I definitely have a problem; part of the truck is cut off.
It's another thing to think about before I take a photo. I need to allow extra room if I wanted it printed a larger size.
If you'd like to read a much better explanation of aspect ratio, visit Exploring With A Camera at Kat Eye View Of the World.