In scorekeeping, what number is assigned to the catcher?

What does "ERA" stand for in pitching statistics? What does a "K" mean on a baseball scorecard? What does a backwards "K" mean on a scorecard? In scorekeeping, what number is assigned to the shortstop? What does a "6-4-3 double play" mean? What is it called when a pitcher allows no hits in a complete game? What is a "perfect game"? What is it called when the fielding team records three outs on a single play?