Machine Screws are a fully threaded bolt available with a variety of head styles. They are mostly used on metal surfaces, with an existing, tapped hole in conjunction with a matching nut.
The shape of the slot in the head dictates what type of screwdriver is to be used. In some cases, a star-type shaped slot is used, this is a torx head, and it is is a security machine screw.
The shape of the head itself varies so that you can choose to have a completely smooth finish, by using a flathead, or a raised or rounded finish by using those head types.