Well Alchemy usually falls into the MISC category, whatever other crafts cant do usually Alchemy falls into doing it. Like they make certain medicines(potions /Hi potions / Echo Drops etc...so to speak), they also use certain herbs and materials to make Silent Oils or Prism powders, and they are also responsible for making certain BST pets with the Philosopher's Stone(yes its in the game). However like every craft they need a crystal and the appropiate mateirals. The way you make things are not any different than any other craft, however your goods that you do will. The good part of Alchemy is that your money makers are usually consumables and are a driving force of its stability in price.

http://mysterytour.web.infoseek.co.jp/ffxi/us/

The link under Guild can show what you can make, and i think Alchemy is certainetly a very useful craft.