On Windows system, for taking screenshot, you must press Print Screen and paste it into Paint to save the picture if no other software is installed.
On MAC OS, this action is very easy to be done with the following instruction:
Hold the keys Command + Shift + 3 at the same time, and then release
2. Capture a part of screen
Hold the keys Command + Shift + 4 at the same time, and then draw a rectangular around the part to be captured.
3. Capture a part of interface (without background and very nice)
Hold the keys Command + Shift + 4, while holding such three keys, you must further press the Space bar, after that, the mouse icon will change to an icon of camera. You move the mouse to any component (e.g.: windows, menu, icon,..), such component will be captured and saved into an image file.
For example we can capture the image of the calculator (widget) as below without background by the third way: