Cloth is used in a variety of cloth and light armors. Even though cloth can be found fairly easily, the next tiers require crafting to obtain.
Leather is used in a variety of light and heavy armors whilst silk tend to be for cloth armor. It is noted that even though silk may seem more logical as cloth it is however considered as leather by the game client.
Ores can generally be obtained via mining ore veins as well dropped from certain enemies. Diggers (e.g. Hemdrills) are also a good source of ores since they feed on minerals underground.
Special materials regroup gems as well as some related items.
Secondary materials are usually man-made in appearance. They are used as secondary parts for crafting and can usually be only obtained via crafting.