hunter role in dungeons

i am new to the game. how do i know when to let my character loose and really start dumping damage on the bad guys? and can someone please explain aggro to me? i have varying levels of damage i can use and dont know how to tell when to step it up. i have been in groups that went through the dungeon smashing through everything in their path and i have been in groups that took them on one at a time. one way is fun and the other is like going to a nine-five job and boring as all get out.