To change the editable area of your image, choose: Image > Canvas Size.

A layer mask is used to hide portions of a layer and reveal portions of whatever layer is below it. You can modify the mask by using the brush tool and painting in either black or white.

Adjustment layers apply color and tone adjustment to your photo non destructively. This means that rather than making an adjustment directly to your photo, you can create a separate layer that will make those adjustments, leaving your original photo untouched.

A drop shadow can give depth to text or elements of an image.