A trek is where we have to locate and interact with 5, 10 or 20 markers picked at random from a list of 186. The full list is at dulfy.net and includes information about how to find them. It only requires one person for each marker.
A Bounty involves finding and capturing 1 (training), 2, 3 or 4 (tiers 1-3) targets picked from a list at random. Usually these targets have a special ability or attack and because of this, the bounties are best done with a large group of higher level characters. Since there is a time limit, if there are enough people to search simultaneously for the targets or even to locate them before the bounty is activated, this helps a lot.