An old saying goes, “Light is the paintbrush of photography,” and it's certainly true. Just as there are many types of paintbrushes, so are there many types of light—soft light, hard light, directional light, and bounced light, to mention a few. Each influences the mood and quality of a photo.
Light also affects such things as color balance, saturation, and contrast. Many times we're able to control these factors. Color balance and saturation are relatively easy to adjust with filters and exposure, but contrast control isn't. One of Photoshop's great strengths is its ability to adjust image properties, and adjusting contrast is a snap with the Auto Contrast command.