World bosses are like outside raid bosses, you start a little raid group and kill them, and they drop purple loot.
There's only 1 per planet, but I'm not sure if there's 1 for every planet.
I killed the Narr Shaddaa one yesterday, he had 560k HP, went with some guildies, dropped pretty nice loot.
He was in the Bonus Quest series area though, at the very end of it.
some planets have more than 1- like hoth.