A screenshot is a picture taken of the entire game screen, including the HUD.
To take a screenshot, press F2 on your keyboard while you are playing in a world. You can change this key binding in settings.
Taking a screenshot generates a 552x414 resolution PNG. The default file name provided is
Mine-Blocks-YYMMDD-HHMMSS.png, where YYMMDD is the date, and HHMMSS is a 24 hour timestamp.