Slick tires, or racing slicks, are special tires which are designed to get the most grip out of a dry, paved track. Slick tires are usually not street legal in any region, so they are typically found on just race cars or show cars.
Why Remove Treads?
Slick tires have no tread because the openings that treads require reduce the overall amount of rubber which comes in contact with the road. Having more of the tire's rubber on the road surface increases the friction between the road and the tire, keeping the vehicle stable at higher cornering speeds.
Racing slicks don't work well on wet tracks because the water on top of the track surface has no room to move out of the way, and the flat tire surface spreads the water evenly between the tire and track surface. When this occurs, little to no rubber from the tire makes contact with the road/track, and all grip/stability is lost.
These tires also don't work well on dirt and gravel surfaces because there is no tread to get a foothold in the unstable surface.