As someone who has done technical writing for many years, I've created a lot of screen captures in my day. When you do a screen capture or screen shot, you basically create an image "snapshot" of whatever is on your screen. Screen shots are extremely helpful if you need to explain how to use software or show someone what was on your screen at a given time.
Although you can buy stand-alone screen capture software like SnagIt that has lots of features, you can do simple screen shots with the tools built into Windows XP. The secret is the PrintScreen key on your keyboard. This key is generally located to the right of the F12 function key on your keyboard.
To capture the entire screen, press the PrintScreen key. Now it seems like absolutely nothing happened, but actually Windows just copied that picture of your screen to the clipboard. You can then paste that picture into another program.
For example, I often use Photoshop to save my screens. If you capture the screen, somehow Photoshop magically "knows" it. So when you choose File|New, by default, Photoshop creates a new blank image that's set to the size of the image on your clipboard. Press Ctrl+V to paste in the screen and voila, there's your screen shot perfectly placed in the Photoshop image. Then you just save the file into your favorite bitmap format or as a native Photoshop file, just as you would with any other Photoshop image.
If you don't have Photoshop, you can paste your screen shots other places as well. For example, after you have the screen shot on the clipboard, you can open up Microsoft Word and press Ctrl+V to paste it in there. However, with that said, I encourage you to save your image files separately first, before bringing them into Word. Getting images out of Word is almost impossible to do well and Word is notorious for crashing when it is filled with a bunch of embedded image files.
If you don't want to capture the entire screen, you can capture just the active window or dialog box. The active window is the one you are working on at the moment. To capture it, hold down the Alt key while you press the PrintScreen key.
If you are really ambitious and want to capture just a menu or show the mouse pointer in your screen shot, you do need to buy special screen capture software. But for quickie screen captures, the PrintScreen key is great.