It is very easy to take screenshot of anything displaying on your iPhone or iPad device. Apple has provided this feature in all versions of iPhone and iPad.
Step 1: Go to web page or iPhone app you want to grab screen from.
Step 2: Once you are on the screen you wish to capture, first hold down the home button and at the same time press the on/off button or sleep button that is on top right of the iPhone.
The screen will flash white and the screenshot is added to your "camera roll" photos on the phone.
Step 3: You can go to the iPhone photos and open "Camera Roll". You’ll see the screen snapshot you just took. It's a high quality image in jpg format which you can email to yourself, so you can save it and upload to your blog or web site.
Note: This works on iPad, iPad mini, and iPod touch as well.