command+shift+4 your welcome it will come up with a crossbow then hold it down and highlight the area, remember google is your friend, even though I have known that for ages i found it on google.
For the iPhone/iPod touch screen shot just press down the home and power/lock button at the same time and the screen will turn white. The screenshot should then be in your pictures.
In addition to Command+Shift+3/4 you can hold the Control key along with the others if you just want to copy the screenshot to your clipboard rather than save it to the desktop.