The rider's aids are both his natural and artificial means of communicating with the horse. The "natural aids" are the rider's legs, hands, weight, and on rare occasion, voice. The "artificial aids" are the spurs, stick (also called "crop" or "whip"), bit, martingale, or any other type of equipment that reinforces the rider's body commands.
The word "aid" is not only used to describe the means of communication, but also the position of the rider's body when giving a command. For example, a teacher might ask the student, "What are the aids for the canter?" The student would reply, "The aids for the canter are the inside leg positioned at the girth, the outside leg behind the girth, and the hands posititioned in an inside indirect rein."