When you look through your camera, the shape you see is this:
This is the typical ratio of all 35mm cameras out there. Quite specifically, this ratio is 2x3.
So you take loads of pictures and you go to get some of these images printed. 4"x6" is the most popular size and it's a good thing, because they conform to the same 2x3 ratio – so you don't lose any bit of the picture.
But what if you want a 5"x7" or 8"x10" print? This is where things get messy.
You see, if you wanted your entire picture squeezed into an 8x10, it would look something like this:
While this doesn't look so bad on a grizzly bear, people are a little less appreciative of this squishing…
You've ordered pictures online and saw this thing, right?
It's a cropping tool and it forces you to choose what part will be clipped off of your picture.
A 5"x7" picture will need to crop off a little on the long side:
An 8"x10" picture will need to crop off more along the long side:
So if you don't have extra stuff around the subject of your picture, what do you do? Something's gotta go. Sadly, a full 2x3 picture can never be an 8"x10" print. An 8"x12" would be a perfect print size (again, a 2x3 ratio) but the history of photography has given us these "standard" sizes which force us into 8"x10" frames.
In practical terms, these are your options when confronted with a tight squeeze:
1) Ideally, you'd order a frame size that works with your picture. In this case, an 8"x12" frame.
2) Crop one or both sides
3) Minimize the image and leave a border (windowboxing)
Does #3 look familiar? It should if you've ever watched a dvd with both widescreen and normal viewing options. The widescreen mode will show you everything, but it'll be a smaller picture and it will have a bar across the top and bottom. The normal mode will fill up your screen but you'll lose the stuff on the ends.
If your photographer ever tells you that he/she simply can't make that picture into an 8x10 print without chopping off someone's head or feet, I hope it will make a little more sense.
If you'd like to read more about the math behind aspect ratios, I recommend you go here.
[I crop ] for the benefit of the pictures. The world just does not fit conveniently into the format of a 35mm camera. - W. Eugene Smith