Sometimes you finished a game level and want to take a screenshot and show off to your friends. If you are suing iPhone or iPad, it is easy. Just press and hold the Home button, then click the power button.
If you have iCloud enabled and have the iCloud client installed on your PC or Mac, the screenshot will automatically push to your PC or Mac through Photo Stream.
To take a screenshot on Android without third party software is different story. It is not for normal user. It is more like for developer. You need to download and install Android SDK on your PC first. Then enable USB debugging mode on your Android device and connect to PC and run DDMS application to do the screen capture.
Now you should understand the difference between iOS device and Android device. One is for everyone, Another is for Geek.