How do I take a screenshot with my iPad, or my iPhone, or my iPod touch?
All you need to do is hold the power and home buttons together and this will capture your screen as an image file and save it to the camera roll.
Skitch for iPad has a nice graphic showing how to do this.
It can take a bit of practice to do this and it is quite easy to turn off the iPad or click the home button to exit the app you are trying to take a screenshot of.
For editing screenshots I like to use Skitch that allows me to add arrows and text.