Extensive use of Krita will almost require you to have some knowledge of layers. Using layers, you can work on one part of the image without touching the rest of it, and most effects are best applied on a layer, instead of on the whole image. Of course, if you do want to apply an effect to an entire image, Krita does offer you that possibility, and there is nothing against it.

The idea behind layers is quite simple. As the name suggests, layers lie on top of each other, and together form the layer stack. The final resulting image is that what you see when looking through the stack from top to bottom. This means that usually the upper layers of your image will have more or less transparency, since you cannot look through a layer which has no transparency. (Krita works with opaqueness instead of transparency. A layer that is 100 percent opaque is 0 percent transparent, and vice versa.) A layer higher in the stack gets applied later than one lower in the stack. For example, if your image contains four layers, numbered from 1 (lowest) to 4 (highest), the effect that layer number 4 adds to the image, is applied to the result from applying layers 1 through 3.

Every image you edit in Krita contains layers. When you create a new image, the layer box (usually shown at the bottom right of your screen, see this section) will contain one layer. The painting and editing you do is then applied to that layer. Once you add more layers, you can choose on which part of the image you want to work, by selecting the respective layer. All further painting is then applied to that layer, until you select another one.

Layers are also an excellent way to check whether adding certain effects (or applying certain image modifications) come out right. Add a layer which contains what you want to try out, and show or hide it with the eye icon in the layer box. You can especially profit from this method if you have multiple effects to check out: show and hide them in any combination, and decide which you like best. And since you can move the layers around, you can also experiment with the order in which the effects are applied.

See the Selections and layers tutorial for a small hands-on introduction.