Block

Blocks are the basic units in Minecraft, and are essential to the gameplay. Together, they build up the in-game environment, and can be mined and utilized in various fashions.

The face of a block is 16×16 pixels, and each block is proportionately one cubic meter. Most blocks are static, although Water, Lava, Portal, and Fire blocks have a shifting pattern for each face.

The world-generated Blocks
Entries marked with a D require additional data values to fully define the block in a Minecraft world. Entries marked with an I have a different ID as an inventory item. Entries marked with a B require additional inventory data to fully define the inventory item. Entries marked with a T have tile entities associated with them to store additional data. Items with IDs in red cannot be legitimately obtained by the player in the game; they can only be obtained by "hacking" (Such as using an inventory editor) or, in multiplayer servers, using the /give command. Items available only in Creative mode are in blue. Items only obtainable through the Silk Touch enchantment in survival mode are in green.

Naturally Generated
Naturally Generated includes blocks that are created through the world seed.

Naturally Created
Naturally Created means a combination of an event that cause a new block to be placed by natural causes, not the player.

Structures
Same as Naturally Generated, but these blocks are created with the "Generate Structures" option enabled.

Additional Reading
In the initial, free version of Survival the player began with 10 TNT blocks and could obtain Dirt, Cobblestone from Stone, Wooden Planks from Wood, Gold Blocks from Gold Ore, Iron Blocks from Iron ore, Stone Slabs from Coal Ore, and white Wool from Sheep.

In Minecraft Classic, the player could build with naturally occurring blocks (except for Redstone and Diamond), and can use Bookcase, Sponge, Coloured Wool, Brick, Obsidian and Moss Stone. Operators of servers could also build Bedrock. Some custom servers give the ability to place Grass and (still) Fluids, too.

When playing the current version, the player can craft a variety of blocks. The Pocket Edition has 2 additional blocks: The Cyan Flower and Camera (which is currently unobtainable).