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⎙ PrtScr. You may have to press the Fn key to activate the ⎙ PrtScr key. This will copy the image on your monitor to the clipboard. You will then need to paste the screenshot into another program, such as Paint or Word. After pasting the screenshot into another program, save the file to save the screenshot. There won't be any indication that the screenshot was taken.   Windows 8/8.1 - You can save a full-screen screenshot directly to a file by pressing ⊞ Win+⎙ PrtScr. The screen will dim momentarily and the screenshot will be saved in the Screenshots folder in your Pictures library as a PNG file. Since you're saving the screenshot directly to a file, you don't need to paste it into another program.  Windows 8.1 - You can share a screenshot of your desktop using the Charms bar. Press ⊞ Win+C to open the Charms bar and then click "Share". Windows will take a screenshot of your desktop and then give you a list of apps that you can use to share it. Alt+⎙ PrtScr. This will copy a screenshot the currently-selected window to your clipboard. You will then need to paste the screenshot into Paint or another image-editing program. Once you've pasted the image, save it to save the screenshot to your hard disk. The Snipping Tool is available in Windows Vista and later. It allows to you select what you want to appear in the screenshot, as well as add basic annotations to the completed screenshot. You can open the Snipping Tool by clicking the Start button and typing "snipping tool". For detailed instructions on using the Snipping Tool, click here. Many games will not allow you to take screenshots using the ⎙ PrtScr key. Instead, many games have a dedicated screenshot key that you can find in the Controls section of the menu. If the game doesn't have a screenshot key, you can use a screen-capture program like Fraps to take the screenshot.

Summary:
Take a full-screen screenshot by pressing . Take a screenshot of the active window only by pressing . Use the Snipping Tool to create custom screenshots. Take screenshots of a game using the game's screenshot key.