defensive ranking

[ngub12]

Just curious. What criteria is used to rate a player's defensive contributions?

For instance, I came across a thread today (and that I have seen in the past) that stated that Jeter is an average def. SS. Im really not a fan of Jeter, however this claim came as a shock to me because I was under the impression that he was in fact a good def. SS.

NOTE: THIS IS NOT A DEREK JETER IS A MEDIOCRE DEF. SS THREAD

Im simply wondering what rubric is used in determing whether or not a player is a defensive liability or asset.

Thanks.