Longbow- i like to use this for general PvE [splinter weapon][barrage] or [broad head arrow] builds cause i like the arc/range/refire rate that it gives
Flatbow-PvE pulling. this is the best pulling bow, same range as longbow(about 1.6 aggro distance on flat ground) but with a slower flight time, it give you a chance to get back behind your tank/melee
Hornbow: i like these if im feeling like running a [penetrating attack][sundering attack] build because a sundering mod, plus the inherent 10% from the bow which stacks and the 10% from the skill is kinda nice
Recurve Bow: Best PvP friend imo. the refire rate/tiny arc size make your bow noticibly more accurate in delicate interrup/condition inflicting situation. which is generally the ranger's niche in PvP (i know there can be many others)
Shortbow: slightly faster refire rate than the recurve, so for spreading poison everywhere you may want to choose this. recurve vs shortbow imo is merely opinion.
hope that answers ur question
http://guildwars.wikia.com/wiki/Bow has more answers/ and a table that compares the different bow types