Copy screenshot of active window to clipboard.
This shortcut will copy the active window's image into the clipboard. You can then paste it to whatever imaging program you like. Note that you cannot paste it directly as a file e.g. onto the desktop.
For beginners: How to press
Alt + Print Screen on your keyboard?
Typically you first hold down modifiers (if exist), and then press remaining key(s) in the shortcut. So,
Press and hold