There's a few different shortcuts:
Command-Shift-3: Take screenshot of screen and save it as a file on the desktop
Command-Shift-4, then drag mouse over an area: Take screenshot of part of the screen and save it as a file on the desktop
Command-Shift-4, then space, then click a window: Take screenshot of a window and save it as a file on the desktop
Hold down control during any of the above and it will put the screenshot in the clipboard instead of a file.