Mechanical turnstiles have existed for around a hundred years. Ones that have three pivoting arms are known as tripods. You see in subways, amusement parks, and arenas. Corporate offices usually have electronic turnstiles with small glass doors, known as “glass wings”, or with paddles. They are used to prevent passage if authorized credentials are not presented. The paddle variety are usually equipped with a bypass to allow crowds unrestricted passage in case of an emergency. Then there are the ones you see with electric beams, they do not provide a physical barrier. The beam variety have a circuit which triggers an alarm if an unauthorized user tries to enter. The most basic type of electronic turnstiles only use a simple circuit alarm and electric beams.
For more information about turnstiles and other access control topics, please check out Thomas L. Norman’s book Electronic Access Control.