There are several organisations in Skyrim that a player will encounter during the game.
The following factions can be joined by the player. In all of the following the player can rise to the highest rank by completing a series of quests.
- Imperial Legion or Stormcloaks (mutually exclusive)
- Bards College
- College of Winterhold
- Dark Brotherhood
- Thieves' Guild
The Elder Scrolls V: Dawnguard
These factions are those that cannot be joined by the player, but can be interacted with:
- East Empire Trading Company
- Silver Hand
- The Synod
About Joining Factions
In general joining a faction in Skyrim does not preclude the joining of another one, so that in one game run most faction quests can be rounded up in one go. That said there are a few cases where this does not apply:
These mutually exclusive quests obviously cannot be played in the same game run:
Though 1. and 2. can be played any way you like, e.g. support the Legion and Vampires.
Concerning joining the Companions and the Dark Brotherhood the situation is less obvious. Joining the former precludes joining the latter. But the Dragonborn can join the Dark Brotherhood first, to later join the Companions.