Shortcuts to copy and paste the entire desktop or the active window

Sometimes you want to take a "snapshot" of the desktop, or perhaps of one of the opened windows: you can easily perform this task pressing the keys Shift+PrtScr (for the whole desktop) or Alt+PrtScr for the active window. This copies the image in the clipboard; you can then paste it in PAINT, for example, or in any other editing program you have: Word, Excel, Power Point, etc.