Cropping is such a hard thing to explain... but I'm going to try!
Cropping is basically the cutting of the photo so that it fits the chosen print size ... 5×7, 8×10, 11x14, etc.
When I take a photo, the original photo can be printed as a 4x6, 8x12, 12x18 or 16x24 without having to be cropped. Since an 8x12 can hold all of the original photo in a print- an 8x10 would obviously have 2 inches cropped out.
When shopping for frames or ordering prints keep this in mind: The most common frame/print sizes will most likely NOT hold all of the original photo! 5x7s, 8x10s, 11x14s and 16x20s will all be cropped.
I always try to account for cropping when shooting, but sometimes there are other factors that play a part in the composition of a photo.
I hope the photos below shed some light on the subject!