What are Gamma Rays ?
Gamma Rays are just photons, "particles of light".
For more about photons, go here.
Gamma-Rays are much, much more energetic than the light you can see, and they are made by different mechanisms than the light you see.
Gamma Rays are at the energetic end of the
Radio waves, infrared, ultraviolet, and x-rays are also part of this E-M spectrum.