A mask is basically:
* Layer Masks are editable selections that play an important role in non destructive editing
* Layer Masks offer flexibility because they can be almost completely treated as a full gray scale image, which allows the use of filters, selection tools, adjustment tools, etc.
* They have a visual strength, meaning that with a short look on the layers you can see how they affect the image
* They offer extra flexibility by being part of vector masks, layer sets, adjustment layers, etc.
* Layer masks allow you to save selections!!
The magic wand also lets you make selections in an easy and more convenient way.
I need more clarification on the erasing thing?
The background is really annoying, just make a layer over it, makes the same effect.