In Photoshop, how do you hide the Cropped areas before cropping them?

Assuming you're talking about PS CS6.

To hide cropped area, first select the crop tool from tool bar then draw a crop rectangle on your image, you'll see a checkbox in crop options with name "delete cropped pixels", uncheck it then crop your image. This will only hide the cropped pixels.

Now if you wish to reveal the cropped area back then go in image menu and click on reveal all. This will reveal the hidden area.

Hope this helps.