Blah probably used the masking tool. That's a good way to do it too cause you can always go back to fix things. If possible do not use marquee or the eraser tools. It's baaaad.
Here's a tutorial...
http://www.melissaclifton.com/tutorial-pentool.html
You can access the Mask tool by pressing Q and using the brush tool. Make sure your foreground/background is black and white (press D to reset and X to switch between the two). Black will be the selection while the white will be the non-selection.