Source: www.epa.gov

 a Alpha particles are high energy helium nuclei. These are highly ionizing particles that are emitted from the nucleus during the decaying process of heavy metals. They, however, are not very penetrating so is usually stopped by as little as a dead layer of skin. But if ingested or inhaled, alpha particles can be very damaging, even deadly.

b Beta particles are high energy electrons. These are also highly ionizing particles that are emitted from the nucleus of the less heavy metals during the decaying process. These are more penetrating than alpha particles. Beta particles can usually be stopped by a layer of clothing but if inhaled or ingested can be very damaging.

g Gamma rays are a massless, chargless waves of energy that are emitted from the nuclei of metals undergoing a decaying process. They are usually emitted along with with alpha and beta particles. Gamma rays are highly penetrating and only be stopped by several feet of concrete or lead.