there are basicaly 4 different types of what you could consider bosses
type 1: Notrious monsters
these are often over camped and only spawn once every 30min-1hr depending on the monster. they drop rare and powerful items but the drop rate isnt exactly the best on some (0/40 on jack woooooo) Notorious monsters are just stronger versions of monsters that you would fight in the area ex: Spiny spipi is just a over sized crawler
type2: (just lost train of thought. go add) o yes Quest and mission
these are your conventional story line bosses the mission ones ussualy dont drop anything good and neither do the quest ones your just killing these to get a better reward latter ie higher rank or rare item from a npc. You ussualy have to spawn these yourself by tradeing a item to something these often have time limits
type3: Burning Circle Notorious Monster aka BCNM
these are fun fights but they often require a exact party ex: 2 beast masters and a white mage for bcnm lv 20 wings of fury. these also drop rare items and have a better drop rate then nms. You have to spawn these yourself also but you have to waitif some one else is in the battle field again time limit
Type4:The god monsters,Ark angels, Shadow lord, Dynamis
these desirve a league of their own and are the hardest fights in the game the god monsters are lv 95 so you need a large group of lv 75s to take these down. the Ark angels are a mission boss fight that can use 2 jobs abilitys includeing 2 hrs. Shadow lord big multi form boss the weakest of the fights in this catagory but still quiet evil if your unprepared. Dynimis is actualy more of a big quest then a fight again a large group of 75s is needed. like i said they desirve a league of their own but if you want to break it down it would be Gods= Nm on steroids, Ark angels, Shadow lord= mission bosses, Dynamis= well i havent realy done this but from what i hear its like a bcnm but with some quest elements thrown in