Guilds are organizations made and run by members. Out of character, their primary purpose is to bring members together and promote character interaction. In order to create a guild, you must have five unique members (not characters) with a designated leader. The leader represents the guild in OOC topics, but this position holds no jurisdiction in character. The guild members are responsible for creating and maintaining their guild's structure. A guild can have their headquarters in any of the worlds.
Guild leaders have the ability to create guild-specific missions. Completing missions will affect your guild's reputation. The way you complete the mission, or if you fail to, will be remembered by the world and reflect your guild. NPCs will react to you differently and your reputation might spread across to other worlds.
Additionally, guilds will have exclusive ability trees. The ability tree is accessible by any guild member, allowing a more uniform and synchronized feeling among guild mates. It is up to the leader of the guild to determine how the guild members can attain such abilities, such as enforcing a rank system, using physical items that grant said abilities, or just allowing them to be used by any member who joins.