Back story is great tool for writers.  It's the trusty sidekick of Conflict (more on Conflict later this month) and can help the reader better understand a great many things.  But use it wisely young Skywalker, or fail you will.  Too much back story (in heaping portions) can kill the momentum of your story.  And we like momentum.  Best to use back story like a spice.  Sprinkle it in to enhance the flavor of the story and tickle the taste buds of the reader.  A dash here, a dash there.  Ooh...that's tasty!