Masking in Photoshop

I've learned enough about masks in photoshop to see how powerful they can be.  But, I can't seem to get them to work for me.  Every time I try to use a mask it doesn't perform in the way that I would expect.  Either is doesn't mask anything at all.  Or it locks me into using only one value.  None of this seems to make any sense.  But, it just isn't working. 

Tonight I tried locking pixels, creating clipping masks, using selections, etc.  None of it worked.  It's frustrating.