Cobblestone Slabs are craftable blocks, crafted using three cobblestone in a horizontal line. Cobblestone slabs behave like Stone Slabs.
They were introduced in Beta 1.3
Used for making lengthy stairs. Two slabs placed on top of each other will create a full-sized block.
Trivia
The Cobblestone Slabs are currently names Stone Slabs ingame even if the look like and are crafted from Cobblestone.
Pre 1.3 update the Cobblestone Slab recipe created a slab that used to resemble the smooth slab. After the update the player has to create stone blocks to create the former smooth stone slabs.