Slab

Slabs were robotic beings, typically androids, used by individuals of various races, typically criminals, as basic slave drones. They tended to travel in pairs. They had a vague form as to not provoke suspicion, but their attitude and internal anatomy were very unusual - they obeyed the commands of their master, disrespected other beings, did not speak and had the same composition all the way through. For example, the Slabs used by a Plasmavore, Florence Finnegan, on Earth (and, due to the Judoon's intervention, on the Moon as well) looked like motorcycle couriers dressed in black, but they never delivered anything and were composed entirely of leather, except for their helmets.

They could be "killed" by sufficiently high levels of radiation, or advanced energy weapons. (DW: Smith and Jones)

Slabs also served the Uvodni general, Uvlavad Kudlak. These Slabs carried Laser rifles and were used to capture the children who participated in Combat 3000. They were also capable of grunting and panting, which they seemed to use as communication (SJA: Warriors of Kudlak)

Slabs were also concrete blocks used by humans, but completely different to the robotic Slabs.

Behind the scenes
Slabs look remarkabley similar to the character of the Stig on the British car show Top Gear.