After a decay reaction, the nucleus is often in an excited state. This means that the decay has resulted in producing a nucleus which still has excess energy to get rid of. Rather than emitting another beta or alpha particle, this energy is lost by emitting a pulse of electromagnetic radiation called a gamma ray. The gamma ray is identical in nature to light or microwaves, but of very high energy.
Like all forms of electromagnetic radiation, the gamma ray has no mass and no charge. Gamma rays interact with material by colliding with the electrons in the shells of atoms. They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays can travel from 1 to hundreds of meters in air and can easily go right through people.
It is important to note that most alpha and beta emitters also emit gamma rays as part of their decay process. However, their is no such thing as a pure gamma emitter. Important gamma emitters including technetium-99m which is used in nuclear medicine, and cesium-137 which is used for calibration of nuclear instruments.