Photoshop is a global phenomena and many of us use or used it without any knowledge. It contains number of powerfull tools that gives users huge power of decisions. In other words there are many ways how to get to the same result in Photoshop. The difference is the question of effectivity and quality. Still some features are common and creates the essence of Photoshop. These are certainly layers and masks. Together they create the core of Photoshop adjustmentes.