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. Water and Lava blocks have a shifting pattern for each face.

Types of Block
There currently are 50 different types of blocks (including Air, still Water, still Lava and blocks which aren't buildable at the moment, see Map Editing (hex) for the full list).

In a standard randomly generated Map you can find the blocks Stone, Grass, Dirt, Water, Lava, Sand, Gravel, Gold ore, Iron ore, Coal, Tree, Leaves, yellow and red Flowers and brown and red Mushrooms.

In the unfinished free version of Survival you can obtain Dirt, Cobblestone from Stone, Wood from Tree, Gold from Gold ore, Iron from Iron ore, Stair from Cobblestone, white Cloth from Sheep and at the beginning you get 10 TNT blocks.

In Creative you can build Stone, Dirt, Cobblestone, Mossy Cobblestone, Wood, Shrubs, Sand, Gravel, Ores, Tree, Leaves, Sponge, Glass, Cloth, Flowers, Mushrooms, Gold, Iron, Stair, Brick, TNT, Bookcase and Obsidian. Operators of servers can also build Bedrock, aka Adminium. Some custom servers give the ability to place Grass and (still) Fluids, too.