Most factions appear to get very similar units, meaning no faction has a real edge over the other.
All of the factions have a leader, and a number of vassals .
As the player you can join a faction of your choosing, rebel against a faction or begin your own kingdom.
Joining a faction is quite simple; it is usually a good idea to travel about in their lands in the early game, performing quests to raise your relationship with the vassals and the liege Lord. Eventually, you may be asked to join a faction as a mercenary; this is quite good early game as the mercenary payments cancel out troop wages, and it gives you a chance to be on the front lines with the faction you chose. You can ask the ruler of a faction to become his vassal, and he will say yes once your renown is high enough and he likes you.
Once you are in a faction, you can choose to rebel against your ruler. Other vassals may join you if they have a low enough relationship with their liege and/or they like you, so it is recommended that you raise your relationships with the vassals before rebelling. You will also require Right to Rule, otherwise nobody would join to your cause and it is likely that your rebellion will be easily crushed.
Creating your own kingdom is similar to that of rebelling, except there is no risk of instant war and you do so in a more neutral manner. This will not prevent a large number of factions declaring war on you.
The factions available at the start of the game are listed below.
The Seven Kingdoms
|The Stormlands||King Renly Baratheon||Yellow|
|The North||King Robb Stark||White|
|The Vale||Lord Nestor Royce||Sky Blue|
|Dorne||Prince Oberyn Nymeros Martell||Orange|
|Dragonstone||King Stannis Baratheon||Dark Pink|
|The Westerlands||Lord Tywin Lannister||Red|
|The Reach||Lord Mace Tyrell||Green|
|The Iron Islands||King Balon Greyjoy||Grey|
|Lord Triston Sunderland||Dark Blue|
|The Nights Watch||Lord Commander Jeor Mormont||Black|
The Free Cities
|Braavos||Sealord Ferrago Antaryon||Hot Pink|
|Norvos||High Priest Caltos||Orange|
|Tyrosh||Archon Qodhrone Omosethes||Pink|
|Lys||Prince Melenai Saskos||Yellow|
|Lorath||Archon Haleq Balar||Light Green|
|Volantis||Triarch Malaquo Maegyr||Light Blue|
|Qohor||Archon Sharako Mortello||Lime|
Invading Factions 
As of version 1.2, there are 3 invading factions in the game, which will spawn either randomly, or after certain conditions have been met. These factions and the prerequestes for them to spawn, are listed below.
NOTE: In version 2.1 The Khizra Khalasar can spawn as The Qevir Khalasar and will start taking over Qohor (needs confirmation)
As of 2.2 the invading factions do not start with any feifs
|House Targaryen*||King Aegon Targaryen||Dark Red||Barrowhall , Crow's Nest , Estermont , Greenstone , Griffin's Roost , Laketown , Rain House , Rainwood , Seawatch , Shipbreakers Bay|
|The Free Folk*||Mance Rayder||Beige||Last Hearth , Greyedge|
|The Khizra Khalasar*||Khal Pono||Beige||Daero Valoti , Rudhy , Selhorys , Zer Sena|