bow strings are upgrades (duh lol). and each one gives you something different.
-there's the 4 elemental bow strings (firey, icey, shocking, ebon)
-poisoner's; one to increase poison duration
-sundering (added armor penetration bonus; +10%/ chance 10%)
-zealous (-1 energy regen/ 1 energy gain on hit)
-vampiric (health degen -1/ heath added on hit 1-5)
i believe thats all of them
as for their uses (in pvp at least), i use an elemental bowstring to get past a warrior's bonus AL against physical damage (can also be used to bypass a necro's bonus AL against piercing damage)
elemental strings in conjunction with its appropriate conjure (lightning, fire and ice, no eath

) gives you a lil added bonus to damage.
i see sundering as useless since you cant depend on the chance to do the added bonus, and 10% isnt all that much either.
barrage + 5:1 vampiric string + big pack of enemies = decent hp gain~