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 image of the screen region you selected will be copied to your clipboard when you release the mouse button. You can paste it in any application by selecting Edit > Paste or pressing Ctrl + V.
Install a screen capture utility such as Snagit from TechSmith
Press Shift-Command-4 . With your pointer turned into a crosshair, click and drag to select the region you wish to capture. When you let go of your mouse button, the screenshot will be saved as an image file to your desktop.
You can only take a screenshot of the entire screen. The methods for doing this depend on the model of iPhone or IPad. See the online apple support pages for instructions.