This demonstrates the effects of cropping on full-frame images. I shoot with a professional digital camera - the 36 MegaPixel full-frame - the Nikon D800.
The images rom my camera will print to a ratio of 4x6 without cropping. However, If you choose a different ratio, such as 3x5, or 8x10, cropping will be necessary. This can result in cutting off information from the image that you want to keep, so you need to make the adjustment carefully when you order. Cropping does not stretch the image or distort the photo; it does not making everyone in it appear tall and thin or short and fat.

But it can result in cutting out someone from the photo if you don't make the crop selection yourself.