How to ..
There are 3 ways to take a Screen Shot, that is taking a picture of all or part of what is visible on screen and they all use the keyboard, Key Commands.
1. Capturing the Entire Screen: hold down the keys Command+Shift and then while holding them hit 3.
A picture of the entire visible screen will be captured and saved to your Desktop and called Screen shot 2010-01-31 at 8.48.17 PM.png. See that the time and date that it was taken becomes part of it's name.
2. Capturing part of the screen: hold down the keys Command+Shift and then while holding them hit 4. Your cursor will change into a cross-hair like the sights of a weapon! Click and drag across the area of the screen you want to capture and on release of the mouse button a Screen Shot will be saved to the Desktop of that area.
This is very handy when grabbing a part of a website or an image on a website that is a part of a Flash movie and can't be saved normally like a regular image from a website. Or is you have paused a YouTube movie and want a shot of the paused image...
3. Capturing an Open Window: hold down the keys Command+Shift and then while holding them hit 4 and then hit the Space bar. The cursor will change into a camera icon. Click the window you want to save and the whole window will be saved as a Screen Shot to the Desktop.