What is "book banning"?

As we wind down Banned Books Week and begin to gather ourselves for the next legislative session, it seems that things are not always what they appear to be.  When we talk about "book banning", there are big visuals of books publicly being taken off the shelves among much fanfare and even destroyed in a very Fahrenheit 451 manner.  Sadly enough, "book banning" is far more covert and insidious than that.