Most digital cameras produce images with the aspect 4:3. Widely used sizes are 2048x1576, 1600x1200 and 1024x768 pixels. Contrary to that, the most common paper size for photos is 15x10 cm (or 6"x4" if you prefer inches), which has the aspect 3:2.
So, if you have a photo from a digital camera and you want it printed at standard size, what happens?
+ = ?
Well, it doesn't fit. If it has to fit without cropping, it needs to be squashed in one direction. That might work for some motives, but most people wouldn't like to be portrayed as 11% shorter than they really are.
With squashing ruled out, that leaved us with two options: Either accept that we can't get standard size or crop the image. If we choose the first, the print service has two choices: Either leave a border or cut the paper to another size. They normally state what they do somewhere on their web page (if they have one).
Some people prefer to get the full image and don't mind borders or sizes like 13.3x10cm (5.25"x4"). Those that don't like such prints can either find a print service that does the cropping for them or do it themselves.
However, unless you pay good money, the print service will just center the crop area on the image. Depending on your photo, that will work fine or it will cut your aunt's head in half. You might want to control the cropping yourself, so that her head can come clean off at the neck instead.
While you're at it, remember that there's no reason that you shouldn't change the scale and the position of the crop area. Don't see cropping as a tedious task, see it as an opportunity to improve composition.
Disclaimer: No, I'm not a professional photographer. In fact, I'm very much a point-and-shoot at my kids & my cat guy. You should have guessed from the photo and the compositions I used above.
I do however feel that I've presented all the available choices above. Should I be in error, I'd very much like an email which points out what I did wrong.