After a nucleus has emitted an alpha or beta particle, the new nucleus often has more energy than it needs. It is left in an unstable, excited state. The protons and neutrons then settle into a lower energy arrangement. This is called nuclear rearrangement.
As the nucleus rearranges, it loses the extra energy by emitting gamma radiation. Gamma radiation is electromagnetic radiation, so it has no mass and no charge.
Gamma emission does not change the number of protons or neutrons. The atomic number and mass number stay the same, so it is still the same element and the same isotope. Gamma radiation is therefore often given out after alpha or beta decay.