In Photoshop, both the levels and curves adjustment layers allow you to control the tone of your image. So which should you use? I'm a curves man myself, but watching this excellent tutorial helped me better understand the use of levels.
Both levels and curves allow you to adjust the black, white, and intermediate gray points of your image to accomplish a variety of editing tasks, including adjusting contrast and exposure. levels is simpler but less powerful than curves in that it is a linear adjustment as compared to a geometric adjustment. Simply put, curves allows for full control over tonal adjustments but it is more complicated.
For those of you getting started with Photoshop or for those of you who don't need or want the added complexity of the curves dialog, Nathaniel Dodson of tutvid expertly breaks down the various uses of levels. It's a great tool for quickly and easily setting your black and white points, adjusting your individual red, green, and blue color channels, adding or decreasing contrast, and adjusting global luminosity levels.
Unlike curves, levels does not allow for specific tonal range adjustments, i.e. adjusting only the highlights or shadows. However, it's a great way to get started on your Photoshop journey as you learn how to push and pull the tonal, luminosity, and color values of your images.