Slabs were robotic beings, typically androids, used by individuals of various races (typically criminals) as basic slave drones for certain things, usually as hired muscle. Slabs were over six feet tall and 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 always obeyed the commands of their master, disrespected other beings, did not speak, cough, or sneeze, were never heard breathing, nor even seemed to feel pain and had the same composition all the way through. They were apparently quite strong, as one smashed through a door with little effort, ripping it clean off its hinges in the process. For them, beings of average strength, such as humans, were easy to overpower, and pin down, as they did it without any effort.
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 all in black including the head, body, arms, hands, legs, and feet, but they never delivered anything and were composed entirely of leather, except for their helmets.
The Slabs' motorcycle leathers were taken from blood victims, killed by Finnegan on Earth.