Feathering is to soften the edge of something (normally for selection and mask). Masking is to cover up something (quite logical isn't it? Hehhee). It applies mostly on layers (except Quick Mask is for selection). If you have two graphics in two different layers, mask the top one to make some part transparent. What's the different from it to eraser? Masking is not destructive. You can easily recover the parts by painting over the mask.
Not history brush.....layer mask is something appear at the side of your layer, and once you create (without selection) the whole rectangle should be white (can see the layer). If you use a BLACK brush to paint on THAT rectangle, it does a transparent effect for the layer.
basically, when you paint black on a layer mask, you are painting OPACITY. When you paint White on a layer mask, you are painting TRANSPARENCY. the effects of the layer will not show through OPACITY, while it shows through TRANSPARENCY. thus you can control the effected area of your layer by painting black and white on the mask.
this is useful if lets say you just want to brighten the subjects face alone.
1. select a levels layer mask
2. brighten up to the desired levels
3. paint everything black on the layer mask
4. paint white over the subjects face on the mask.
transparency on the mask on the face means that the levels that you have set effect only the face of the subject...
hope these detailed desciptions help. and of course, layer masks do more than just levels. from curves, hue/saturation, gradient map... etc... go experiment....