We talked about ionizing radiation, discussing alpha particles, beta particles, and gamma rays. Alpha and beta are both particles, with mass, but gamma rays are better thought of as waves. What can we say about that?
Gamma rays are electromagnetic radiation, which is another way of using the word “radiation.” You know a lot about electromagnetic radiation, because light is electromagnetic radiation. Gamma rays are the same stuff as light, but with extremely high frequency and low wavelength. Above, some waves are sketched, showing what frequency and wavelength mean.
Electromagnetic radiation with high frequency is dangerous, because it has high energy (because it’s "waving" so fast). This makes it ionizing radiation, i.e. a dangerous kind of radiation.