Even easier with an Apple. Hold the Command, Shift, and 4 keys down simultaneously to get a gun sight icon on the screen. Drag the icon over the image you want to capture holding down your left mouse button. When you release the mouse button, you'll hear a click and the image will be saved to the desktop. If you want to change the format, open the image, and use File, Save As to select a different format like a JPEG.
You can do the same thing with Windows, but I can't for the life of me remember the keys you need to use.
ETA: Just realized that Vista and Windows 7 has a Snipit tool that does much the same thing
1. Click on to the round Windows logo in the lower left hand corner.
2. Click on All Programs.
3. Click on Accessories.
4. Select Snipping Tool.
5. The mouse pointer will change to a crosshair. Select any or all of the screen.
6. Immediate following, a window will show that will have your target rectangle inside of it. Click on File and then Save As to save your file.
