Right now there are three NPC guilds in the game, which you can join - all offering a unique skill line for your character:
- The Fighters Guild is pretty much what you expect them to be. They've taken a massive contract to fight back Molag Bal's Dark Anchors and all Daedra.
- The Mages Guild is pretty much what you expect them to be as well. They're trying to preserve knowledge throughout the times of war.
- The Undaunted are new in the series. They're a group of adventurers trying to prove their worth and they're a bit like a group of posers. They want to fight the biggest, strongest, meanest creatures and win.
Unfortunately, some guilds didn't make it right into launch , but they're scheduled to be added at a later date, most likely later this year:
- The Dark Brotherhood will be added later. They're still quite the same as in the previous games and will most likely offer contracts and bounties.
- The Thieves Guild will be back as well. They didn't release any details so far, but it's quite likely that its introduction will also introduce a crime/stealing system.
Also, there are quite a few other groups in the game. Some you'll be able to join, but they won't necessarily grant you direct bonuses or skills. Warning: spoiler ahead!
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The Eyes of the Queen are a network of spies working outside the Aldmeri Dominion's laws (as far as necessary) and subject only to Queen Ayrenn herself.
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The Morag Tong are quite similar to the Dark Brotherhood, although they're quite focused on Morrowind.
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The Shadowscales are a group of Argonians bringing the gift of Sithis to those worthy. Another group of assassins (is there some pattern? ;)).
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The Beldama Wyrd are a circle of witches from Glenumbra. They're pretty much classic druids, but only women.
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The Companions you've met in Skyrim appear in ESO as well, but you're not able to become one of them.
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And probably many others I just forgot to mention right now... this list is definitely incomplete.
Both Vampires and Werewolves (PvE) will spawn in high-level areas, and their attacks have some chance of infecting you with the relevant disease. Once infected, you will have a new quest that you must complete to either become the monster you wish to be, or be cured of the disease.
You can not be infected by the Vampire disease if you are currently infected with the Werewolf one, and vice versa.
Vampirism Effects (more details)
You'll unlock a new skill line you can make use of, you'll also start to have these effects at various stages, until you feed on a human opponent (PC or NPC), which will reset it to "stage 0".
- Up to 30 mins:
- 30 to 60 mins:
- -25% Health Regeneration
- +50% Fire Damage Weakness
- -20% Health Cost to Vampirism Skills
- 60 to 90 mins:
- -50% Health Regeneration
- +50% Fire Damage Weakness
- -40% Health Cost to Vampirism Skills
- More than 90 mins:
- -75% Health Regeneration
- +50% Fire Damage Weakness
- -60% Health Cost to Vampirism Skills
Werewolf Effects (more details)
You gain access to a skill, Werewolf Transformation that does pretty much what it says on the tin. While transformed, you'll have access to a new line of skills to unleash your inner puppy, and there are some ill effects of course -- such as a passive ability that increases damage taken from abilities in Fighters Guild skill line, +50% to all poison damage.
PvP Infection
Players can also infect other players with their condition.
For Werewolves, the skill Bloodmoon can be used once every seven days, acting as a rate-limit. For Vampires, the equivalent skill is called Blood Ritual. You have to use these skills at the ritual sites, so it's unlikely you could be turned against your will. Both require level 6 in the respective skill trees.
Best Answer
The option is a bit hidden; obviously so you don't do it by accident.
It's quite easy once you know it: