The Guilds of D'ni were organisations, each with a specific responsibility in D'ni society. After the abdication of King Kerath and until the Fall of D'ni, they were also the government of D'ni with several members from each Guild serving on the D'ni Council.
Major and Minor Guilds
The 18 Major Guilds were considered the most important. They existed for centuries, and their members could serve on the D'ni Council. The Guilds were extremely exclusive, as only the sons of previous Guild members could join.
There were many other Minor Guilds, that were smaller, less important. Unlike the Major Guilds, membership was not restricted and it was generally much easier to become a member, which was the reason of their establishment by King Mararon. Members of minor guilds could not serve on the D'ni Council.
Each Guild had their own unique insignia and color. Guild robes were dyed the color of the Guild, though it is possible that less colorful robes (such as Cho's uniform) also existed.
Guilds also had their own Guild halls where members lived and worked, though members with families could also own their own houses. Guild halls could be single buildings, or entire complexes of buildings for the larger guilds such as the Writers or Maintainers.
Members were expected to abide by Guild rules and have a sense of pride for their Guild. Violation of Guild rules could result in expulsion, as in the case of A'gaeris.
Even though each Guild had their own traditions, each Guild (presumably) operated under the same basic hierarchical structure. Members were inducted into service at a very young age. When they completed their training they attained the rank of Guildsman. Each Guild had numerous Guild Masters who had served for many years, and each Guild was lead by a Grand Master.