Bedrock

Bedrock is a block indestructible by survival means.

Obtaining
Bedrock cannot be broken using any tools. It also cannot be moved by pistons or destroyed by explosions that are present in survival gameplay. Bedrock can only be obtained by the creative inventory and commands.

Natural generation
Bedrock comprises the bottom-most 5 layers of the Overworld in a rough pattern, although to the 4th layer is predominately flat bedrock with no gaps, rendering the very bottom 5th layer inaccessible. In the Nether, bedrock comprises both the top and bottom 4 layers in a rough pattern. At the top of the Nether, bedrock prevents the player from going past layer 127. It can also be found at the sides of the Nether in the console edition. Bedrock in the Overworld and the Nether always generates in the same pattern no matter what seed is used.

In the End, defeating the Ender Dragon spawns a portal with a bedrock frame. Individual bedrock blocks make up the natural ender crystal structure and the bedrock remains after the destruction of the crystal. As of 1.9, bedrock additionally forms as part of the end gateway portal, created when the dragon is killed, and the bedrock portal is spawned when you enter the end, not when the dragon is defeated.

Usage
Bedrock's primary usage is as a solid unbreakable block. It also prevents mob spawns and is the only solid block that beacon beams can shine through.

Additionally, when in the End and set on fire, the fire on bedrock will never go out naturally, similarly to netherrack.

Trivia

 * In the console editions, only the lowest layer (and highest in the Nether) of naturally spawned bedrock is unbreakable.
 * It is possible, but rare, for ore to be found encased in Bedrock.
 * In Minecraft 4k, there is bedrock on the bottom of the map with no texture.
 * Strongholds can cause a bedrock not to form within its bounds.