The Twelve Gehenna Passages

The Greek word Gehenna is used twelve times in our New Testament. Typically, most English Versions translate the word as hell. Is this a good rendering? What do the Scriptures of the New Testament mean when it speaks of Gehenna? How should the word be understood? Often the phrase unquenchable fire is used when speaking of Gehenna. Does unquenchable mean non-ending? How does Jesus' sacrifice of Himself for sin impact any of this?