In ancient times, when we mostly used a 4-3-4 alignment, everyone spoke more or less the same language. You knew about defensive tackles, ends, strong, middle, and weak linebackers, corners, and free and strong safeties.
Nowadays, the further you get from the DL, the more exotic things get. In a 4-DL set, you can use 2 "true" LBs and one hybrid - either part LB and part safety, or part LB and part DE. Likewise, your secondary can employ 2 CBs covering the outermost receivers, and then any number of safety/cornerback combination players. He could be a CB with short area pass coverage duties who's spun down to run force like a safety.
The duties and responsibilities changing mean most DCs now create new-ish terms for those "non-traditional" roles on a defense. What one coordinator calls a "jack" might be a "will" or "spur" to another.
With wider alignments and more complex decision trees by the offense, defensive coordinators have to place a premium on athleticism. Getting strong, fast guys on the field while still respecting the easiest play in football strains the old terminology.
