Slabs are effective if you want to keep away mobs considering only slabs and glass prevent mobs from spawnig. This includes passive, neutral, and hostile mobs. They also can be used for compact staircases. They come in all types of wood, cobblestone, stone, quartz, brick, and sandstone.
You cannot place a slab on another slab of different type
Slabs used to be destroyed by only pickaxes, even wooden ones