Option - Shift - 3 will save a screen shot your entire screen to your desktop. Option - Shift - 4 will bring up a little cross hair thing so you can define what you want in the scren shot. Option - Shift - 4 + Spacebar will give you a little camera which you can use to take a screen shot of only one window.
There's probably more... I know there's someway to just copy it to the clipboard, but I can't remember it...