Full face helmets cover the entire skull and have a protective section over the front of the chin. Such helmets usually come with a visor that generally swivels up and down to allow access to the face. Many full face helmets include vents to increase the airflow to the rider's head.
Open face helmets have a rear that covers the back of the head, but lack the lower chin-bar of the full face helmet, and do not necessarily include a visor. An open face helmet provides the same skull protection as a full face helmet, but less protection to the face and chin areas. Helmets without visors allows insects, dust and wind to impact the face and eyes, which may cause rider discomfort or injury unless used with goggles (goggles should meet Australian Standard AS 1609-1981).
Flip-up helmets are a hybrid between full face and open face helmets. They are also sometimes termed "convertible" or "flip-face" helmets. When fully assembled and closed, they resemble full face helmets by incorporating a chin-bar for absorbing frontal impacts. The chin bar usually pivots upwards to allow access to most of the face, as in an open face helmet.
Dual purpose helmets are a hybrid of on-road and off-road helmets. They have clearly elongated chin and visor portions, a chin bar and a partially open face to give the rider extra protection while wearing goggles and to allow the unhindered flow of air necessary during this type of riding. The visor allows the rider to dip their head and provides further protection from flying debris during off road riding.