The Passive Voice

Everyone talks about passive voice, but what is it exactly? How can it be made to work to benefit a piece of writing and when should it be avoided all together? When is a piece too passive? 

Passive voice occurs when the subject of a sentence is neither a do-er or a be-er. The subject exists, but not focused on. The real emphasis happens- when it is the most important- to the thing acted upon, or when the actor is not important.