They drop four ash when broken, and have a smoke-like particle effect above them. Players and mobs walk more slowly on this block, just like soul sand. The resulting ash can be used in a furnace resulting in 2 smelting processes per item making them as efficient as coal. Also, you can craft coal by putting it into a crafting table in a 3x3 pattern.
You can also use ash to create Gray Dye. One block of ash will create two bowls of Gray Dye.