Taking screenshots can be an effective tool for teachers for a number of reasons. Basically, whatever is on your screen can be captured and saved as a picture. For example, a social studies teacher can pull up google maps on the iPad and take a screenshot of a state or country to share with the class as a visual aid. Apple has made it very simple to take a screenshot on the iPad (as well as the iPhone for that matter).
All you have to do is press the home button and the power button at the same time. Done. The screen will flash white (to mimic the idea of a flash) and you can access your screenshot from the Photos app.
Here are some examples:
So there ya go! There are ways to crop, rotate, and edit images which I will cover soon.