Guilds are tradesman's organizations built around specific synthesis skills. Joining a guild allows access to its various services, which are helpful in several ways. First, going to the guild and training is required to raise the level of an associated skill. Guilds also offer more direct assistance in certain tasks. When attempting a difficult item synthesis, you can go to the guild and get help from an expert, which increases the chances of success. Finally, guilds also maintain shops that sell items they make, or raw materials for making those items yourself.
Crafting... it's a controversial system which is quite advanced in Final Fantasy XI. With crafting, you can either be the person who sits on the ground while the people gather in awe wondering how much Gil you will have when the items you've recently crafted sell on the Auction House. Or... you can be the other guy in the corner who is upset because they had just spent their last 5,000 Gil on crystals, which can be found on the ground in broken pieces.
Harvesting, Excavating, Logging and Mining (H.E.L.M.)
Fortunately, there is a way to be a good crafter with a cost effective method.
Commonly referred to as "farming", crafters can find their materials through other means than purchasing them. Check out the H.E.L.M. section, listed here are locations for some core materials used in crafting.
However, if crafting isn't your forte yet and you are hard up on cash, the following methods can also earn the willing adventurer enough coins to survive.
Crystals are crystalline objects that contain the life force of the world. Control of the Crystals amounts to control of corresponding bits of the world, and since all three major nations have expansion on their minds, the adventurers of each nation are roaming far and wide in search of Crystals to expand the power and territory of their homelands. Crystals can be found in the monster-filled wild lands of each country. Areas where adventurers travel are ringed by border guards installed by the government of the nation that lays claim to that territory in order to aid their countrymen. From the border guards, you receive a "signet," which is an enchantment that makes it easier to find crystals or pick them up from fallen bad guys. The enchantment wears off after a little while, though, so it's wise to get those crystals while the getting is good.
There are eight types of crystals, each corresponding to a particular naturalenergy and related attribute. Each crystal has its opposite, easily discerned from the listing above, and each elemental energy also has a particular quality that describes the manner in which it might prove useful. Fire embodies combustion; water embodies dissolution; earth embodies joining; wind embodies separation; ice embodies coagulation; thunder embodies decomposition; light is the force of reproduction; darkness is rot and decay.
All these different qualities play a role in the business of manufacturing items. The key to creating magical weapons, armor, and other objects is unlocking the different energies contained within the crystals. Gaining levels in item synthesis accords a character more impressive titles in their chosen trade, and more importantly, it reduces the chance of costly failure in item synthesis. That's important, because the crystals and other raw materials necessary to make powerful items may take a fair amount of effort. The actual business of synthesis is relatively simple -- once all the items are collected, all you need is a place to cook. Mix the materials together, add the appropriate crystal as a catalyst, and if luck is with you, presto! A new magical or otherwise useful item. Maybe you can keep it for yourself, but one of the main reasons to specialize in a trade, naturally, is to use your products to engage in commerce with others.