Photoshop has a huge array of tools and features, and many of them have some degree of overlap between each other. Two of the most fundamental and powerful tools you have at your disposal in the program are Curves and Levels, and this excellent video tutorial will show you the difference between them and how you can use them for editing your images.
Coming to you from Aaron Nace with Phlearn, this fantastic video tutorial will show you the difference between Curves and Levels in Photoshop. Curves and Levels are two of the most powerful tools in the application. You might think of these tools as being used for adjusting things like exposure or the shadows or highlights, but their capabilities actually go much further. By separating the adjustments into the individual red, green, and blue color channels, you can also quickly and easily perform color correction or color grading. I personally prefer to use Curves, but be sure to watch the video above for the full rundown from Nace.
If you would like to continue learning about Photoshop, be sure to check out "Photography 101: How to Use Your Digital Camera and Edit Photos in Photoshop With Lee Morris!"