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

What is a "walk-off" hit? What does "RBI" stand for? What does "ERA" stand for in pitching statistics? What does a "K" mean on a baseball 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? What is it called when a pitcher allows no hits in a complete game?