Guilds in Final Fantasy XI
Guilds are where you sign up for different crafting skills. Crafting allows you to utilize the items you acquire from drops in battle to create items you need in the game. The higher the crafting level, the more unique items you can synthesize. Crafting is another great way to earn gil, as you can buy the ingredients for less, and sell for more.
When you cap your level (usually every 10 levels) you can take a Rank Exam at the Guild Headquarters in order to level up further. Below are the lists of the Guilds in Vana'diel and some recipies available in each rank.