Guilds exist to create an extra dimension to your characters. There are those who are interested in fighting, those in money, those in evil doing, and of course those interested in nothing at all. The guilds give you a guide to how your character MIGHT act in some situations, and generally guide how you MUST do combat in the Realms. A wizard would fight with spells, a fighter with cold steel; it all depends.
The way the guilds aid you is in cheap advancement of skills. For information on how skills work read the skills concept information. Guilds provide cheap training in skills that are appropriate for that guild. For example, a fighter can get cheap training in all fighting skills.
There are a plethora of races to choose from in this world, each with distinct behavioral attributes, certain abilities, and complex social structures. Most organizations only accept members of a certain race, such as the Dwarven Smiths, or any of the Drow Houses or Elven Clans.
Dawn has 8 distinct races to choose from, as well as 2 additional half-breed races. There are drow, elves, orcs, humans, bastet, dwarves, gnomes, and hobbits. The half-breed races (half-elves or half-orcs) have no organizations to call their own. However, they have the option to choose their organization from the stronger side of their ancestry. The races domain's job is to add to, fix, maintain, and update the organizations of each of these races.