Levels allows you to correct the overall tonal balance of an image, broken down into shadows, midtones and highlights, while Curves allows you to adjust the individual points in an image’s tonal range.
Many Photoshop users favour one or the other for adjusting images, but the truth is that they should both have a place in your image-editing workflow.
Both of them offer you the option to save presets so the you can apply the same adjustments to multiple images or image assets, and both allow you to drill down into colour channels to make individual adjustments.
Where they differ is that generally, Levels is used to make tonal adjustments to the image as a whole, and while Curves can do this too, it also allows you to make the adjustments to individual tones and colours within the image without altering other parts of it.
Advantages of Levels
Advantages of Curves
What do you use? Levels? Curves? Both?