NPC guilds are organizations focused around a set of quests. Often, these guilds will usually have a theme such as crafting, weaving, thieving, merchanting etc. These guilds also have merchants that will sell items relevant to the guild's theme. There are regional guilds and national guilds.
There will be several regional guilds for each guild theme. These regional guilds report back to national guilds. Monst guilds will require a certain skill level, hunter title, or monster before allowing you to begin taking quests. Once the player reaches the requirement for the guild, the can receive quests that are related to the guild's theme. After completing all the quests and tasks the guild has to offer, players will receive items unique to that guild. They will also receive a small discount when buying from that guild's merchant.
After completing all the quests for all the regional guilds of one theme, players can go on to enter into the naitonal guild. There are similar requirements to begin accepting quests for the national guild with the exception that they are usually much more difficult to achieve. After completing all the national guild quests, the player will receive items unique to the national guild. These items are soulbound. They will also receive a greater discount from all the regional guild merchants as well as a small discount from some nonguild merchants.