Need or Greed will give players in the group two options.
When an item is looted (anyone can activate the loot) a window will pop up for everyone with two buttons
Need and Greed
If its an upgrade for you, you press Need
If its not an upgrade or you dont want it you press Greed
The Computor then rolls a dice for every one that selected Need and the highest roller gets the item
If no one selects Need then the computor rolls a dice for everyone who selected Greed and the highest roller gets the item
Loot threshold is the quality the loot will need to be inorder to activate a roll, if you set it to uncommon then only green+ items will be rolled on and anyone can loot white items
Master loot should bring up a window for the person set as master looter, wich will give them the option of sending it to anouther player or kepeing it themselfs
Ive not tryed it in STO tho so it may just mean only that person loots.
I would think Free For All would be if the loot drops for the team, who ever in the team gets to it and picks it up first gets it.
The only reason I don't think that is because in OB, when I was a group leader in a Open Instance Mission (4 in group and instance), I tried Free for All and saw gold and silver loot objects... they should have all been gold if that was the case.