Zerger = someone who runs ahead killing mob to mob without hesitation or patience or regard for the placement of other party members. They "Zerg" into battle with little other then killing what is in there way as there motive or tact.Thus Zerging is the risky and reckless attempt to complete an objective in a fashion that has little forethought or true strategy and relies on deficiencies in opposing defenses and surprise to exact a costly victory (or failure).
(*edit This does not include running past archers that dont move which is a zerging tactic as well .. let it be noted also that many non zerging parties also run past archers)
Exploiter = someone who takes advantage of in game design such as "PoP back" and other such game flaws or game mechanics to achieve there goal with ease.
( this does not include running past archers which is a general zerging tactic, this also does not include sneaking which is neither zerg or exploit but tactical repositioning this specifically relates to running past mobs you have already aggro'd until they POP BACK or cannot otherwise follow you with exception ONLY to stand still ledge archers that dont move to follow there targets)
ex·ploit (eks′plo̵it′; also, and for v.usually, ek splo̵it′, ik-)
transitive verb
to make unethical use of for one's own advantage or profit; specif., to make
profit from the labor of (others) without giving a just return
Thus the unethical use of the POP back feature for ones own advantage or profit (exp or loot) is exploiting by definition of the word.)