The shortcut CMD+SHIFT+3 will take a snap of whole screen. CMD refers to Command key to either side of your keyboard.
The shortcut CMD+SHIFT+4 will create a handle to crop a screenshot area. This is the most unknown thing I am going to say. Once you press CMD+SHIFT+4 , you can press Spacebar and it will take screenshot of the window you click on. The screenshot window will be greyed.