Barrier

A barrier is an invisible block which appears transparent, but has mixed properties.

Obtaining
Barriers can only be broken by withers in survival.

Barriers are not listed in the creative inventory, and can only be obtained using the commands, such as.

Pocket Edition
Barriers can only be obtained in the inventory via hacks.

Barriers spawn naturally as the border of Old maps until the height limit. It can be broken by a Nether Reactor if inside the world, however on the outside, the barrier will simply cut off the structure.

Usage
The block in the player's inventory shows up as a red box with a slash through it, but when placed is completely invisible.

Barriers are transparent to light, but interact with blocks and mobs like a solid block. All dependent blocks can be placed on a barrier, such as trapdoors or redstone, and it will suffocate mobs. Mobs cannot spawn on barriers.

In creative mode, if a player is holding a barrier block in their hand, all placed barrier blocks display the barrier icon as a particle. This effect is client-side, and if a player holds a barrier block in survival, the particle won't show up. Barriers cannot be pushed by pistons.

Pocket Edition
When placed, barriers grant an x-ray effect. Barriers also drop a shadow. Projectile weapons directed at the block will strangely pass through, and they will be blackened as they are in a light level of 0. Mobs are also blackened, and will suffocate.

If a semi-block, like a stairs (with it sloping down towards the block) or fence, the side facing the barrier will be severely blackened, with a light level of 0 as in the barrier.

Trivia

 * The "No" symbol used is actually an inverted version of the ISO standard.