Most image editing involves applying or removing color. By selecting a part of the image, you can limit where color can be applied or removed. When you select a tool and move the mouse pointer onto an image, the appearance of the mouse pointer changes to reflect the chosen tool.
With many tools, an icon of the current tool is shown along with the arrow. For paint tools, an outline of the current brush is shown, allowing you to see exactly where you will be painting in the image and how large of an area will be painted.