I'm not really a noob ARC/BRD but ...

[Krindor]

The game has 2 common classifications for skills, Weaponskill/Spells and Abilities. Skills classified as Weaponskills/Spells all share a common cooldown(Called recast in this game) the standard is 2.5 second cooldown often reffered to as global cooldown or gcd. In comparison skills classified as abilities do not share cooldown with any other skill, neither ability nor gcd. These are commonly reffered to as oGCD or of global cooldown. Taking your example of heavy shot and bloodletter, heavy shot is classified as weaponskill and therefore shares a cooldown with other weaponskills. But Bloodletter is classified as an ability and therefore only has its own cooldown(Does share a cooldown with rain of death but that is stated in the ability). What this means is that you can use heavy shot and then bloodletter straight after without waiting.