You will frequently find yourself wanting to copy things from outside applications into Krita or copy things inside Krita itself. If you are working inside of Krita, this is done from the main menu Edit > Copy (Shortcut: Ctrl + C) and Edit > Paste (Shortcut: Ctrl + V.
If you are working outside of Krita, you can use the PrintScreen button (Windows) or Command + Shift + 4 (OS X) to copy your monitor display area to the clipboard. If using the Internet, you can right-click an image and select the Copy option. If you want to paste the clipboard data into a new image, from the main menu select Edit > Paste into New Image (Shortcut: Ctrl + Shift + N).
There are specialized versions of the cut and paste actions. By default, Krita will soften the edges of your cut and copy selections. The sharp options will remove the softening (anti-aliasing) to give crisp edges. These are accessed from the main menu Edit > Cut (sharp) and Edit > Copy (sharp).
Take your selection and copy all of the layer contents. By default, Krita will only copy one layer. With Copy merged, the clipboard will copy everything that is visible in the selection. This is convenient when working with a lot of layers. Access from the main menu Edit > Copy merged.