Guilds will effect what your character may earn in Quests, Most offer a Bonus Item along with the Quest reward as Hyda only offers bonus Golji.
You may only join 1 Guild at a time, whichever Guild you join will grant you a percentage off of what its about (Most grant a 20% discount, Hyda boosts Gojli earned by 15%).

Here is a list of Available Guilds:
Mythic - Pets (20% off), Artifacts (reward)
Hunters - Monsters (reward), Artifacts (reward)
Inventor - Artifacts (reward) , Materials (20% off)
Vulture - Swords, Katana, Rapiers, Sabers (20% off/reward all)
Lion - Axes, Spears, Maces, Hammers (20% off/reward all)
Mako - Rings/Staffs (20% off), Necklaces (reward), Earrings (reward)
Chimera - Sai, Shuriken, Hands/Claws (20% off/reward all)
Eagle - Guns, Bows (20% off/reward both)
Dragon - Metal Armor (20% off/reward)
Serpent - Fabric Armor (20% off/reward)
Slither - Whips, Luxuries (20% off/reward both)
Technical - Relics (reward), Materials (20% off)
Hyda - Golji (15% increase in Quests, Post Quests, and Gauntlets)

Note 1: Hyda, Frey, and Fame are the only guilds that effect Gauntlets.
Note 2: After you invent 5 new items related to your Guild you may switch, PM Dasweld to switch.
Note 3: The longer you stay in a Guild the better they will reward you.

If you get into every guild and switch (when you get the ok to do so), you can open up the Frey Guild which increases your xp/sb/ep gain by 10%, and the Fame Guild which Doubles your AP gain (2ap would equal 4ap).

Mastering a Guild will also open up new guilds (Mastering requires that you Invent new items that your Guild covers, for example to Master Vulture you would have to Create 5 swords/katana/rapiers/or sabers); To Master Hyda you have to Spend over 1000g.

Final Guild : All Bonuses (Must Master All Guilds)
Lore Guild : Double SP Gain (Requires: Mythic, Hunter, and Mako mastery)
Smith's Guild : Inventing Costs 1g (Requires: Vulture, Lion, Mako, Eagle, Inventor, & Chimera).
Frey Guild: 10% xp/sb/ep gain from Quests/Post Quests/Gauntlets/Post RP (Requires that you simply join each of the Available guilds once)
Fame Guild: 2x AP gain from Quests/Post Quests/Gauntlets, Post RP (Requires that you simply join each of the Available guilds once)

Please note that Guilds will not take effect in the Marketplace (PC owned shops) forum. You will have to purchase items there at their set price, Store owners will not gain extra golji from sold items if they are in the Hyda guild.