[Updated 9/6/17]
If you've ever asked a question on these forums about some issue you were having on your device, chances are one of us asked you to post a screenshot. Here's how to do it.
Taking a Screenshot
This can vary slightly between devices, and typically requires Android 4.0 or above. The standard way on most Android devices is to
press and hold the Power and Vol Down buttons for about half a second. (On Samsung devices, it's Power and Home, or you can do a palm swipe:
How to take a screenshot with a Samsung Galaxy S4 | Android Central.) You should hear a camera shutter tone, and the current screen image will show as having been captured. The image is stored in the directory /storage/sdcard0/Pictures/Screenshots (not the DCIM/Camera folder), but it should be readily viewable in the Gallery or Photos app.