While it is not possible for the end user using Windows Phone 7 to take screenshots on the phone, Microsoft has enabled that functionality for the Windows Phone 8 users. Did you just finish personalizing your start screen, and want to share your work of art? You can capture your screen and send it to your friends.
1. Press the power key and the start key at the same time.
2. Tap Photos > Screenshots.
3. Tap and hold the screenshot you want to send, and tap share
Update – in windows 10 mobile, you take screenshot by pressing power and volume down button at the same time. The combo for windows phone 8.x no longer works.