If you are referring to my "exotic bounties", each time you complete a bounty you have a small chance (1%?) to get an exotic bounty from the postmaster.
These are long quest chains with multiple requirements that reward you with an exotic weapon.
Here is an example:
http://destiny.wikia.com/wiki/Toland%27s_Legacy
This is one of the ones that I have. The annoying part is that they all seem to have a crucible requirement.
I tend to only have 30 minutes a night to play or so, and don't want to devote a month to getting good at Crucible, so I'm debating trashing these bounties.
There are, of course, other (easier) ways to get exotics. Either through drops in raids or buying them from Xur with Strange Coins.