The screenshot feature has been included in a number of Samsung devices and now you can join in if you have a HTC. Before you start you will need to make sure you are running Android version 2.3.4. To check your version you will need to go into the phones settings screen and then find the "About phone" option and scroll to "Android version". As long as it says 2.3.4 (or higher) you're ready to go.
Now on any screen you want to take a screen shot on press the power button and the home shot and the phone should take a screenshot of it's self with out any need to download an app or even rooting the device.
One of the key uses of this is for guides though another, growing use, is to use the images to post them on forums to show off problems. For example if you have a specific symbol showing and want to know what it is you can post a screenshot and make things easier for people trying to help you.