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 named "Stone Slabs" ingame, even if they do 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.