FFXIV has a lot of fun and interesting dungeons to explore and our guide goes over everything you need to know in understanding the duty finder, roulettes, and more.

Exploring dungeons in Final Fantasy XIV: A Realm Reborn is fun, necessary, and educational (in the ways of raiding). It's a requirement of the main story quests to complete a variety of dungeons, while it's also necessary if you plan on ever hitting level 50 on your first class (or level 60 with whatever class you want) in a timely manner. Dungeons in FFXIV work similar to other games, but there are a few things that are important to go over.

Group Party Size, Undersized Parties, and Level Sync
In FFXIV, a light party is a group with a minimum of 4 players, a normal party is 8 players, and a raid is a collection of up to 32 players. Each dungeon has a "requirement" of the amount of players required to complete the dungeon, but that's the maximum. Each dungeon also has a level maximum, in which if you enter the dungeon in the full sized party (like through the duty finder) then everyone will sync in level with the dungeon.

If you enter the dungeon with fewer than the required players, level sync will not occur. This is useful for dungeons that free companies are attempting to run someone through in order to farm tomes for someone who hasn't finished it yet or to just help them. For instance, in Castrum Meridianum, a required dungeon to complete, you can enter with up to 7 players and players will retain their level 51-60 stats, health, damage, etc. making the content trivial. If you enter with 8 players, then level sync will occur.

Level Sync
High level players can enter low level dungeons through the duty finder, duty roulettes, or by grouping up with a full party and sync to the level of the content. This works the same way level sync for FATEs work. Everything about your stats is equalized to the level of content, unless you play on the Chinese servers, which as of patch 2.4 do not use level sync. Level sync works on skills, as well, so you can only use skills you'd have at the same level.

The reasoning behind this is so that high level players can help low level players through content without ruining it for them.

Treasure Coffers
Throughout a dungeon, there are optional treasure areas to complete for wealth and prizes. Some treasure coffers have a low chance to give out some random gear from the dungeon, but otherwise throw a few potions your way. Other treasure coffers have a specific loot table, usually affixed to a "mini-boss" or a timed-event that must be slain or completed in time for the treasure coffer to spawn and be accessible.

Room Seals
All boss fights, except for trials, will seal the room after 15 seconds. Generally, cinematics will prevent players from entering the boss fight room, or should only trigger after crossing the glowing purple haze that marks one room from another.

Dungeons vs. Raids vs. Trials vs. Guildhests
Dungeons are usually composed of 3 or more bosses spread out throughout a lengthy dungeon crawl with trash monsters and various mechanics to access each of the bosses. Dungeons can go up to 8 players.
Raids are multiple party dungeons that generally require a lot of players (16+).
Trials are 4 to 8 player single boss fights.
Guildhests are 4 player training battles.

Roulettes
Daily there is a variety of dungeon related roulettes in the duty finder that offer additional rewards for a random dungeon. Expert, High Level, Low Level, Trial, Main Scenario, Guildhest, and Frontline. They all pull from dungeons from their listing (low level is 49 and below for example). They provide a nice daily reward, in the form of tomes and gil. For the low level roulette, it has a huge exp bonus.

1 tank, 1 healer, 2 DPS per group and iLvL Restrictions

That's the group composition. You only need two players to do most duties, but each duty is unique. Some dungeons require specific ilvl (usually level 50 and above dungeons).

As a final tip, /return turns you back to the entrance.