A mudflap is typically made from a flexible material such as rubber that is not easily damaged by contact with flying debris, the tire, or the road surface.
Mudflaps can be large rectangular sheets suspended behind the tires, or may be small molded lips below the rear of the vehicle's wheel wells.
Mudflaps can be aerodynamically engineered, utilizing shaping, louvers or vents to improve airflow and to lower drag.
[1] While some flaps are plain, in the colour of rubber, many contain company logos, other art or sometimes advertisements.
Supercomputing technology applied to the problem of semi-trailer truck drag has helped to validate such aerodynamic improvements.