On this super quick video tutorial portrait photographer Nathaniel Dodson (from the website tutvid) demonstrates how to easily invert a mask in Photoshop and why is this a good idea in some cases.
cmnd/cntl i is the way to invert your mask (from white to black and vice versa). This is very simple but also very powerful as this allows you to either apply the mask (and any changes to it) to a part of your image or all of it or somewhere in the middle (think of invert mask as a way to save you a lot of time when you want just specific parts of the image selected or de-selected).
You can check out other videos we have published demonstrating the use of layer masks in Photoshop.
Bonus video – Changing Layer Opacity Trick Photoshop CC