There is no open-world PvP in FFXIV. Instead, PvP in this game falls into 3 categories:

- 1v1 duels in the dedicated duel arena at the Wolves' Den Pier in La Noscea (from level 1)
- Frontlines: 24v24v24 large-scale campaign (from level 50)
- The Wolves' Den: 4v4 or 8v8, either ranked or unranked (from level 50)

To participate in any PvP activity (except duels), you must be at least level 50 and have met the requirements for a particular event to appear in the Duty Finder before you can participate. All PvP is accessed through the Duty Finder. There is no cost to participate; however, if you leave a PvP duty instance before it has concluded, you will be unable to participate in PvP duties again for 30 minutes upon exiting (Rated The Feast locks you out of all content for 30 minutes).

Once you meet the requirements, it is a simple matter of queuing up in the duty finder for the event and tier you want to participate in. You can join alone, or with a party. Players in the same party will be on the same side in the PvP duty.


Duels:

The Wolves' Den Pier has a large open area where players may challenge each other to duels. You can participate after completing the level 1 quest It's Time to Duel. This means that dueling is open to all Disciples of War and Magic at any level can duel.

Frontlines:

There are four modes across three different areas, the Carteneau Flats (CF), Seal Rock (SR) or the Fields of Glory (FoG). Only Shatter seems to be populated, as per patch 3.5.

A random frontline battle can be completed via the daily Duty Roulette, or you can queue in the PvP section of Duty Finder.

The Wolves' Den:

The unrated Wolves Den plays host to two types of arenas:

The Fold: The original Wolves Den encounter, this is a 4-on-4 Last Man Standing battle. Two teams battle it out and the match ends when all members of one team are KO'd at the same time. - no-one does this any more, as of patch 3.5

The Feast: In this arena, two teams of 8 battle it out to collect the most medals. Each player starts with 100 medals, and you drop half of your medal total when you are KO'd. The match ends when one team reaches the target number of medals. If neither team accomplishes this before the time runs out, the team with the most medals will be the winner.

There are also Rated options for 4v4, where your performance will be ranked. These follow the same rules as for the unranked Feast, but players will earn a rating that goes up and down based on the outcome of the matches. As you earn rating points, you will ascend through five ranking tiers, which will be shown on your PvP Profile. Ratings for solo queue and party battles are kept separately.

Gear and rewards:

As you participate in PvP battles, you earn PvP experience to increase your PvP rank.Each PvP rank awards Action Points which can be spent to unlock PvP-exlcusive actions and traits. If your current class/job is not level 60, you will also earn normal experience points from non-level 60 battles.

In addition, each PvP event has a selection of items and equipment with stats tailored towards PvP. See the specific content's page for the rewards associated with it. Some PvP equipment may have a rank requirement to equip, and if PvP gear has Materia slots, the materia placed in them will only take effect in PvP content. Outside of this restriction, however, PvP reward gear can be used in any content, with stats equivalent to other gear of the same item level.

There is a selection of special mounts, titles, and gear that are earned from PvP-related achievements. You can also use Wolf Marks (PvP currency) to buy other things, like orchestrion rolls and minions.


Below are some resources that go into more detail on how PvP works in general and also further info/tactics for The Feast:

PvPaissa: pvpaissa.com
Resource for all your PvP needs. Includes PvP guides and rankings for The Feast, etc.

Gamerescape PvP wiki info

The Feast Guide 1

The Feast Guide 2

Guide to the GARO event (until December 2017)

Info shamelessly stolen & adapted from Gamerescape (link above)