What does a "K" mean on a baseball scorecard?

What does it mean when a batter "hits for the cycle"? What is a "walk-off" hit? What does "RBI" stand for? What does "ERA" stand for in pitching statistics? What does a backwards "K" mean on a scorecard? In scorekeeping, what number is assigned to the shortstop? In scorekeeping, what number is assigned to the catcher? What does a "6-4-3 double play" mean?