In this context, All Terrain Vehicle means a one-rider three- or four- wheeled off-road thing rather than a road vehicle/passenger car or truck with augmented suspensions, big tires, and winches. The windshields for regular-sized vehicles are covered in the other articles of this series, although specially-strengthened windshields are available for true off-road conditions.
Much the same considerations as for snowmobile windshields apply: their main purpose is to deflect small objects and to divert airflow. Since on an ordinary ATV the operator is above the handlebars and body of the machine, a windshield that protects him/her from all the airflow would be something like three feet tall and not very durable, unless it was braced with steel or aluminum to such an extent as to interfere with the operator's vision. ATV windshields concentrate on protection against missiles from the ground or from the machine directly in front: in particular, they guard the operator's hands.
All the remarks above about selecting, buying, and installing snow machine windshields apply to ATV's as well. Clubs and dealers will give you the best ideas as to what windshield to choose for your ATV.