Natural Blocks spawn randomly throughout the world and generally cannot be crafted.
Building Blocks are commonly used to construct buildings and structures.
Manufactured Blocks do not occur naturally and must be crafted.
Utility Blocks help you survive and/or create cool stuff.
Mechanisms are items that players, other items and redstone circuits can interact with.
Ores are minerals that can be mined with a pickaxe.
The Nether is a seperate world that can be reached by constructing a portal.