First of all you need to know a little bit about cameras and their internal sensors. Each camera has a different imaging sensor that captures the image. Most, if not all 'consumer' cameras and DSLR's have what they call a 'cropped' body or image sensor, as opposed to the higher end cameras, which are considered 'Full Frame' Cameras.
The main difference, for the average person, is when it comes to printing your photo. Obviously the higher end cameras have much better sensors, functions, and much greater differences than this post will go into. So, I am only going to mention the photo size difference here. There is a big misconception out there that photo labs are cropping the photos and consumers are led to believe that 5x7's and 8x10's are standard sizes. In part thats true, but for the mid-range DSLR's and point-and-shoot cameras, because of their 'cropped body,' the true proper sizes, if you want the image you see to print out fully on photo paper are as follows: 4x6, 8x12, 10x15, 11x17, 20x30, but good luck finding an 8x12 or 10x15 frame.
Now on the other hand in 'Full-Frame' cameras, which most are around $1200 and up, what you see is what will print out in the more 'standard' sizes like: 5x7, 8x10, 11x14, 16x20, 20x24, 24x30.
I believe the reason being is that way back when, when Kodak first started printing images, the cameras they used were essentially 'Full-Frame' film cameras and obviously only photographers that could afford them used them, and could charge whatever they want. When the technology came along to make the cameras smaller, they obviously had to make the sensor smaller so it was more affordable for the average consumer. This in turn effected the way the photos printed. Because the 'standard' sizes were the only sizes used for so many years everyone just sticks with it.
So lesson learned here, if you are shooting on a 'cropped body' camera, make sure you leave enough room around the edges to be able to print the standard sizes.
‘What makes photography a strange invention is that its primary raw material are light and time’
- John Berger