Crafting
Crafting is having a shop make an item: many shops make things as well as sell them, each from a fixed list. A recipe needs materials, most also take a sum of eons, and the shop’s page lists everything it can make.
Material trees
Most materials are themselves crafted, so an item’s page shows the whole chain: an Ancient Spear opens five levels deep and comes out at 360 Basic Log, 76 Iron Ore, 46 Shiny Coin, 12 Buckets Of Water and a Spear, which a shop sells. A tree stops at anything a shop sells, at gathered materials, cores, gear frames and jewelry bases: those are bought, mined or farmed, not crafted again.
Some items can be made more than one way, at different shops and for different money. Every option is listed, each with its own tree.
Breakers
The reverse also exists: a breaker takes a finished piece and returns a base part. That is where Gear Frames and Jewelry Bases come from, and why a material tree stops at them rather than unravelling a finished item you would have had to make first.
From the game’s help window
From the in-game help (F1). Where it disagrees with the sections above, they are current.
You can craft special items by going to certain npcs and aquiring items for them.
What do I generally need for crafting?
Well generally you will need monster parts to craft certain items, but npcs might ask you to get other items as well as paying a bit of gold.
What can I craft?
Well you can craft anything basically, mostly special weapons though. Go to certain npcs and see what they can craft for you.
Where can I craft items
That you need to explore around and find out.