Here are instructions for taking a screenshot of part of your screen in Windows and on a Mac.
Press “Windows + Shift + S”. Your screen will appear grayed out and your mouse cursor will change. Click and drag on your screen to select the part of your screen you want to capture. A screenshot of the screen region you selected will be copied to your clipboard. You can paste it in any application by selecting Edit > Paste or pressing Ctrl + V, just as you’d paste a full-screen shortcut taken with the Print Screen key.
Press "Shift-Command-4." It turns your cursor into a crosshair, which you can drag to select a portion of your screen to capture. Release the mouse button or trackpad to take the shot.
You have a number of other options after hitting Shift-Command-4:
Press and release the space bar: The crosshair turns into a little camera icon, which you can move over any open window. Click on your desired window to take a screenshot of it. A screenshot captured by this method features a white border around the window with a bit of a drop shadow.
Press and hold the space bar (after dragging to highlight an area but before releasing the mouse button or trackpad): This locks in the shape and size of the selection area but lets you reposition it on the screen. It's very handy if your initial selection area is off by a few pixels; just hold down the space bar to reposition it before releasing the mouse button to snap a screenshot.
Hold down the Shift key (after dragging to highlight an area but before releasing the mouse button or trackpad): This locks in each side of the selection area made with the crosshairs save the bottom edge, letting you move your mouse up or down to position the bottom edge. Without releasing the mouse button, release the Shift key and hit it again to reposition the right edge of your selection area. You can toggle between moving the bottom edge and right edge by keeping the mouse button or touchpad engaged and pressing the Shift key.