Beta rays, also known as beta particles, are high-energy, high-speed electrons or positrons emitted by certain types of radioactive decay. They have a negative charge and are much smaller and lighter than alpha particles. Beta rays are more penetrating than alpha particles and can travel several meters in air and several millimeters in human tissue. They can be stopped by a few millimeters of aluminum or a few centimeters of wood or plastic. Beta rays are commonly used in medical imaging and radiation therapy.