| When we talk about cropping a photo, generally we're talking about removing part of the image. However, it is also possible in Photoshop to extend the crop box beyond the boundary of a photo, adding additional space in the image. This is especially likely, I find, when you are rotating the image along with cropping, such as to correct a horizon that isn't straight. In order to get the crop you want, you may end up with a triangle of space where one corner of the crop box extends outside the image area.
Under normal circumstances without the Content-Aware feature, the extra space added by a crop box that extends beyond the boundary of the image would be filled with the current background color defined on the color picker at the bottom of the toolbox.
If you turn on the "Content-Aware" checkbox on the options bar for the Crop tool, and the crop box extends beyond the boundary of the image, then when you crop the image the empty space will be filled using the Content-Aware technology.
So, for example, if you are rotating the image to straighten the horizon, you might find that a crop that remains within the image area will cut off part of a cloud formation you particularly like. If you extend the crop box to include the full cloud formation, the crop box would extend outside the image and you'd end up with (by default) a white triangle in that area of the photo.
With the "Content-Aware" checkbox turned on, that empty space would instead be filled through the use of the Content-Aware technology. If the surrounding area of the photo is relatively simple, this will generally produce very good results. If the area is a bit complex, with textures and shapes that will make any duplication obvious, this feature can be problematic.
So, for simple situations I recommend using the Content-Aware feature for the crop tool. For more challenging circumstances I recommend leaving this option turned off, and then using a combination of the Content-Aware Fill command along with the various image cleanup tools to fill in the empty area in a way that will not be problematic in the final image.
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