The article below is just a basic introduction to the topic. For something more advanced, written by pros and a fantastic leveling guide, my recommendation would be this Elder Scrolls Online Leveling Guide. I could not find any other guide that comes even close in terms of the impact it had on me.
All characters in The Elder Scrolls Online will be able to choose and join as many guilds as they wish. Guilds in this respect refer to NPC faction-based guilds that are not the same thing as player-created guilds. There will be four main guilds in TESO when the game launches: Fighters Guild, Mages Guild, Vampire Guild and Werewolf Guild. More guilds will be added in future updates.
While a Dragonknight will be able to join all four guilds if they choose to, it makes more sense to pick and choose guilds that will benefit that particular class most since the rewards for each differ and the skill trees for each are also completely different. It also makes the most sense to specialize early on while leveling. Joining certain guilds can also help influence conversations with NPCs while playing through the main story.
For a Dragonknight, joining the Fighters Guild makes more sense than joining the Mages Guild, but both do provide benefits. The Fighters Guild skill line has more to do with crowd control abilities, area-of-effect melee abilities and attacks and abilities that affect Undead and Daedric enemies. The Ultimate ability from this skill line may be useful for Dragonknights, especially, as it provides a boost in AoE utility.
The active abilities can also be useful for tanking Dragonknights. Circle of Protection, for example, is one ability that may help when tanking and providing support for your dungeon group. Fighters Guild also allows aspiring PvP Dragonknights to accept bounty quests in Cyrodil. The vendor rewards for the Fighters Guild will also prove to be quite useful, especially while leveling and beginning endgame gearing.
While the Mages Guild does not seem like a typical choice for the battle-hardened Dragonknight, there are still a few reasons for joining. The active abilities and Ultimate ability are all quite excellent if the Dragonknight is utilizing Magicka or staff weapons in their arsenal. For Dragonknights that are sticking with pure melee combat and steel, joining the Mages Guild is more of a personal and optional character progression choice.
Both the Vampire and Werewolf guilds may have a few useful active abilities and passive abilities for Dragonknights. Joining either respective guild will allow the Dragonknight to temporarily change into either a Vampire of Werewolf. Both of these toggled abilities may provide useful for soloing gameplay purposes. It’s also likely that the vendor rewards for either guild will be useful.
The article above is just a basic introduction to the topic. For something more advanced, written by pros and a fantastic leveling guide, my recommendation would be this Elder Scrolls Online Leveling Guide. I could not find any other guide that comes even close in terms of the impact it had on me.