The Blocks are the things we mine and put,

The bricks in 2D

There are 25 Blocks, They appear randomly on your level, The ones who most spawn is Grass, Wood and Others that is "natural"

You can retexture the Blocks by: Mods/TexturePacks, Click on one of the images in there, put on Paint and when you finish editing click on Save As...

There are just 25, But when the game update, Then there shall be alot more blocks.

As of 1.3.1 update, new blocks have appeared, but a bug cause the old blocks to be pushed out of the inventory.