How can you take screen shots on an android device?

I was on the damnyouautocorrect website and I saw a few screenshots of android devices. I was wondering how it's possible to do so. Somebody told me that there are some applications on the phone that allow you to do it. If so, does anybody know the applications? Free ones preferably. I also heard that it requires a download onto your computer and plugging in your android to the computer with a usb cord. I'm not exactly sure how to go about doing that, so if you do know it would be helpful information to me. Thanks(: