The concept of image styles is that you can upload a single image but display it in several ways; each display variation, or <em>image style</em>, is the result of applying one or more <em>effects</em> to the original image. As an example, you might upload a high-resolution image with a 4:3 aspect ratio, and display it scaled down, square cropped, or black-and-white (or any combination of these effects). The Image module provides a way to do this efficiently: you configure an image style with the desired effects on the <a href=":image">Image styles page</a>, and the first time a particular image is requested in that style, the effects are applied. The resulting image is saved, and the next time that same style is requested, the saved image is retrieved without the need to recalculate the effects. Drupal core provides several effects that you can use to define styles; others may be provided by contributed modules.