Barrier

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

Obtaining
Barriers cannot be broken in survival mode.

Barriers are not listed in the creative inventory (similarly to command block) and can only be obtained using commands, such as.

Pocket Edition
Due to the lack of commands in Pocket Edition, barriers can only be obtained in the inventory via hacks and look almost like a sheet of stone when held.

They 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. Barrier blocks will not be shown if the particle setting is set to "minimum". Barriers cannot be pushed by pistons.

Pocket Edition
When hacked in, they are are called ".name<" when selected.

When placed, barriers grant an X-ray effect, as they are opaque yet lack a texture. They can also be seen through in third person mode, and this can be exploited to reveal caverns and other underground structures. Barriers also cause 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 stairs (with it sloping down towards the block) or a fence is next to the block, the side facing it will be severely blackened, with a light level of 0 as in the barrier itself.

Trivia

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