HomeI can stress enough how important masks are to learn in Photoshop. Chances are, if you don’t already use masks you’ve either never heard of them or can’t be bothered to learn or care. Let’s explain why you should care. So you’ve been working hard at this photomanip for hours, you’ve got the effects you like. But something is just off. Oh no! A part that you erased earlier shouldn’t have been. You can’t undo that far back so you have to either go with it or (shudder) start over.
This is where masking comes in. If you want to erase partially, or wholly, part of an object, you can add an alpha mask to it (or even to groups!) and paint in grayscale onto the mask which changes the layer’s transparency in that spot. It’s a lot like using the eraser tool but…not. You see, the eraser tool is what we call a destructive manipulation and masks are non-destructive.
Let’s jump into an example. Here’s a butterfly. 
It looks like crap.
So let’s erase the background and make it blend better. You probably whipped out your eraser tool, or more accurately; kept reading this paragraph and did not touch Photoshop at all. But fret not, I will humor the destructive method first.
Here you can see that I used the eraser tool to get rid of the background and draw a new gradient behind it with a cloudy texture. THIS IS BAD. No, bad designer! Here, take a look at this useful little button:
That rectangle with a circle in it applies an alpha mask to the layer (you can add multiple masks) and as you can see, the layer’s mask was altered instead of the beautiful butterfly itself this time.
For you old hats out there, don’t laugh at the newbies reading this (or this poster) because there was a time when you didn’t know about the mask feature and erased all your images manually too!
