Many Photoshop users learn to use the Levels dialog and then never progress to learning how to use the Curves. Let me give you several reasons why learning to use the Curves will make you more of an expert with Adobe Photoshop.
In digital photography editing, both the Levels and the Curves are used for one of the single most important editing tasks: image tone control. Whenever you have a photograph that has less than perfect contrast, i.e. the contrast is either too low or, conversely, too high, you need to use some form of tone control. Both the Levels and the Curves allow you to alter the black and white end points of the tonal range in the image. Adjusting the end points allows you to change the global or overall contrast of a photograph; the method and affect is the same whichever of the two commands you choose to use.
Both commands also allow you to change the appearance of the intermediate–or grey–tones in-between the black and white end points. However, the important difference between the two commands is that a Levels adjustment proportionally changes all of the tones in the tonal range; whereas the Curves allow you to choose which portion of the tone scale you wish to adjust. The Levels is a linear adjustment but the Curves are a geometric adjustment (in fact, this difference is in the names: levels and linear, curves and geometric). This is a massive difference, and using Curves rather than Levels will elevate your editing skill to a professional level.