Minecraft wiki entry for Custom_Blocks, associated text: 1 Tutorial: Adding a simple custom block
2 Replacing Existing Blocks
3 Removing Existing Blocks
4 Using Images From A Texture Pack
5 Different Kinds Of Block
6 Sub Blocks (for mods)
7 Available Block Types
7.1 Air
7.2 Solid blocks
7.3 Water
7.4 Log