Most factions are a group of NPCs that you can gain notoriety with. At certain levels of reputation with them you can purchase rewards like crafting recipes, weapons, armor and other assorted wares. Below is a list of all known factions.
Player faction is one of two factions that all players belong to. Each faction has its own major city, and cannot group with members of the opposite faction. If you pvp a exception is made when entering warfronts as you will occasionally be placed as a mercenary for what would normally be your enemy faction. Your player faction is chosen during character creation and your choice of faction will alter the availability of Races available to you.
Guardians are the chosen of Telara's gods, the Vigil. When the tyrant Aedraxis called forth the power of the Plane of Death and devastated Telara, the Vigil chose the most powerful souls taken by the Shade to be the archons of the next age. A golden age awaits, but Telara must first be saved — by deed, by example, or by the sword. Its people must be redeemed, its corruptors cleansed. Only then will the Guardians be imbued with the divine and ascend to a higher level.
The Defiant do not care whether the gods abandoned the world or not. To them, this entire planar convergence disaster is the gods' fault in the first place. Indeed, as soon as things went from bad to worse, the gods suddenly and inexplicably disappeared.
So while the Guardians run around Telara building temples, sticking their noses in everyone's business, and desperately praying for a miracle, the Defiant plan on actually delivering one, even if it means being branded as heretics and operating outside the laws of the land.
Planar factions are factions you gain notoriety with after killing Plane bound creatures or completing planar encounters. You can't reach higher than Honored notoriety with these factions currently. See below for a list of the factions and their elemental plane.
Regional Factions (also called Ally Factions), are factions based in a specific zone. These factions can provide rewards for your notoriety with them to help you level up faster. Many ally faction quartermasters sell Armor Runes. Below is a list of each zone's Ally Faction.
Warfront Factions are factions you gain notoriety with when you complete objectives in Warfront. Below is a List of warfronts and their corresponding Factions. PVP Factions refer to your city faction requiring you to do either daily's or weekly's in order to progress them.