Block

Minecraft is composed of and built entirely of blocks. You dig through blocks, you build with blocks, your character is made of blocks; they are everywhere.

A face of a block is 16x16 pixels, and each block is estimated to be one cubic meter. Water and Lava blocks have a shifting pattern for each face.

Types of Block
There are currently over 55 different types of blocks (including Air, still Water, still Lava and blocks which aren't buildable at the moment, see Blocks, items and data values for the full list) and assuming Cloth as a one of a kind (counting every color).

In a standard randomly generated infdev Map you can find the blocks Stone, Grass (or Snow), Dirt, Water, Lava, Sand, Gravel, Gold ore, Iron ore, Diamond, Redstone, Clay, Coal, Tree, Leaves, Cactus, yellow and red Flowers and brown and red Mushrooms.

In the initial free version of Survival the player began with 10 TNT blocks and could obtain Dirt, Cobblestone from Stone, Wood from Trees, Gold from Gold ore, Iron from Iron ore, Step from Coal, and white Cloth from Sheep.

In Classic mode the player can build with naturally occurring blocks, save Redstone and Diamond, and can use Bookcase, Sponge, Coloured Cloth, Brick, Obsidian and Mossy Cobblestone. Operators of servers can also build Bedrock, aka Adminium. Some custom servers give the ability to place Grass and (still) Fluids, too.

When playing the current Alpha, the player can craft a variety of blocks not naturally occurring on maps, including Stairs, Workbenches, Paintings, Bookshelf and Furnaces. Redstone and Diamond can be found in the lower areas of maps, and Obsidian forms when water touches a spring lava block.

Also in the current Alpha, the Mob Spawner block may naturally occur in a Dungeon.