There are other reasons to crop! With many print sizes available, you might want to focus in on a certain segment of the image. In the photo here, I've highlighted a square format on a rectangular picture.
The "crop box" that will let you choose which area you want in the picture is a red outlined box that matches the size of the print you want (see above for an example).
The print you receive will match the area contained within the red "crop box" or, in the example to right, the see-through red square.
Please make sure you crop your photos to capture the good stuff or the stuff you think is good!