Block

Blocks are the basic units in Minecraft, and are integral 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 16x16 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.

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

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

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

Additional Reading
There are currently over one hundred different types of blocks, including Air, still Water, and still Lava, which cannot be built, various types of Wood, various colors of Wool, various types of Pressure Plates, Slabs, Stairs and Double Slabs. See Data Values for the full list.

In a standard randomly generated Map, you can find Air, Stone, Grass, Snow, Dirt, Water, Water Source, Lava, Lava Source, Sand, Sandstone, Gravel, Gold Ore, Iron Ore, Diamond Ore, Redstone Ore, Lapis Lazuli Ore, Clay Block, Coal Ore, Bedrock, Ice, Wood, Birch Wood, Wooden Planks in Abandoned Mines Pine/Spruce Wood, Leaves, Cactus, Sugar Canes, Vines, Pumpkins, yellow and red Flowers, brown and red Mushrooms, dead and living Shrubs and Tall Grass. In dungeons (added in Seecret Friday Update 2), you can also find Cobblestone, Moss Stone, a Mob Spawner, and Chests, even though cobblestone and chests are usually made by the player. Moss Stone, Cobblestone, Wooden Planks, Fences, Pressure Plates, Bookshelves, Crafting Tables, Stairs, Black Wool, Iron Bars, and Furnaces may also be found in the randomly generated NPC villiages that were added in 1.8, and Stone Brick, Mossy Stone Brick, Cracked Stone Brick, Iron Bars, Bookshelves, Wooden Planks, Chest, and Cobweb can be found in the strongholds and abandoned mines also added in 1.8. Obsidian can be found naturally, but it is not part of the random generation; instead it is created when naturally occurring lava source blocks and water come in contact. Cobblestone can also be found naturally, and is formed with the same circumstances as obsidian, except that it is formed when water and moving lava contact. Dungeons are also built of cobblestone in parts.

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 Classic mode the player can 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 can 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 not naturally found in maps, including Crafting Tables, Bookshelves, Furnaces and more. Redstone, Diamond, Gold and Lapis Lazuli can be found in the lower areas of maps, and Obsidian forms when water touches a spring lava block.

Netherrack, Glowstone, Gravel, Soul Sand, Nether Brick, Nether Brick Fence, Nether Brick Stairs and Nether Wart can be found in The Nether, which the player can only enter by creating a Portal.

The Pocket Edition has 2 additional blocks: The Cyan Flower and Camera.

Trivia

 * When placing a block in 1.0.0 with a slightly laggy computer, it may place two blocks, and when you remove one block, it removes them both. This also happens with doors.