The default loot method option is group loot. Everyone gets their own random chance to get an item, chosen from that boss's loot table and their own selected loot specialization. (By default, your loot spec matches your current specialization, but you can change it by right-clicking on your portrait in the upper left.) Then, if someone gets an item with a lower item level than what they currently have equipped, they can give the item they just got to someone who could also have looted it.

If you found a group with the Looking For Dungeon interface, the thing that matches you with four other people at random and teleports you to the dungeon, group loot is the only option. Group loot is also the only option in a pickup group. However, if you're in an all-guild raid (or maybe a few non-guild members can come, but very few, something like no more than 20 percent of the raid, I don't know), you can set it for master looter. This works basically like it always has: items are on the boss, the master looter chooses to give it to people in the raid based on whatever system they've agreed to. The only change to master looter in recent expansions is that the amount of loot bosses drop scales with the number of people in the raid.