It's called a screenshot. If you're on a PC (which you probably are) if you push the "Print Screen" key on your keyboard, it will create an image of whatever's on your screen at that time. You can then open up your image editing software application of choice and Ctrl-V or Edit Menu -> Paste to get the image off the clipboard. Then you can crop it to just include whatever you want like Boostn did.

If you're on a Mac, you can hold down Command-Shift and the 4 key, and then you get a little crosshair cursor. Then you can click and drag to just select a portion of your screen, and once you let go of the mouse button, you hear a little shutter sound and a PDF version of the screen image is saved on your desktop. There you go!

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