Guilds are groups of people who gather together for a common cause. Guilds often pay their members for completing jobs, and these jobs are furthering the Guild's cause.
There are many Guilds, each one has their own speciality, such as Magic or Summoning.
Dragon Slayer's Guild: The first Guild ever made. It is used for training aspiring Dragon Slayers new Magic, Sword and Archery techniques. The Guild was "disbanded" when a massive group of Dragons attacked the Central Dragon Slayer's Guild Hall. There are a few members remaining, but those who remain live in seclusion to avoid any Dragons that may be looking to wipe them out.
Warrior's Guild: A Guild full of both powerful warriors and aspiring knights. They often get jobs from towns to clean up trouble monsters, but they have been recruited into the military, and have helped other Guilds with their duties.
Mage's Colony: The Colony was once a famous and powerful organization, having multiple bases in every region, including the Northern Continent, but was completely disbanded when a massive group of Dragons attacked the Central Dragon Slayer's Guild, crippling their main supplier. About a year after this event, the Dragons then attacked hundreds of Mage's Colony bases, destroying any architectural evidence of their existence. Years later, the Colony was rebuilt, and rebuilt stronger, at that. The recovery became a major News story. More information on the Colony can be found here.
The Peacekeepers: A Guild of the strongest warriors in all of Zeterra, and it's neighboring Realms. The Peacekeepers are the first to arrive at a battle, and the last to leave. They are ruthlessly strong, killing the "unworthy" and banishing them to oblivion. The Peacekeepers also have the favour of the Gods, giving them even more power. Their leader, Lady Mauvitaa, is the strongest. In battle she uses Goliath Armor, made from a combination of all known metals. Her followers use Titan Armor, only slightly weaker than Goliath Armor.
The Order of Luna: The Order of Luna is a Guild who harness the sheer power of madness. Their attacks cannot be predicted, due to their strange fighting style. Their leader, Lady Luna, is the absolute pinnacle of madness, able to warp the area around herself to shred apart outsiders. Just being near her makes one go insane. Their group focuses on converting people to follow the Whisper and Brura-Nol.
The Thieves Guild: The Thieves Guild is a massive web of guilds, businesses and public figures who steal, cheat and lie to gain something, usually being money. Almost every town and city in the realm has a root or two of the Thieve's Guild. It's hard to say who's really in charge, since the Guild is so secretive.
The Order of the Tower: The Order of the Tower, now commonly called the Tower Knights, are an organization of powerful and secretive mages and warriors. The Tower Knights subtly influence Zeterra, socially, politically, militarily and so on. They attempt to move the realm forward, even if it seems as if things have come to a standstill. All of their members are trained to use the Tower Knights' signature tools: Grandswords, Grandbows and tower shields. These items are all far larger than anything a normal person could handle. And, depsite their size, the Tower Knights are also trained to be many times more agile than the most prestigious acrobat, trained to jump higher, run faster and attack faster than many of their enemies could aspire to. The Tower Knights, despite being secretive, are easily spotted in a crowd. They wear sleek, black armor, with small slits for eyeholes. They also often tower over a normal person, partially because of their height, and partially due to their helmets, which often have decorative pieces of metal. They also possess great magical power, and combining it with their already impressive abilities, make them capable of handling small armies alone, or with a partner. It is currently unknown how many members there are.