...are mostly know as the waves that is emitted as a by-product of radioactive processes. They are at the top of the spectrum that humans can reproduce.
From this point forward, waves start to be very dangerous because they interact on the nuclei in atoms.
A lot of things in space: super-/hypernovas (exploding (massive)stars), supernova remnants (caused by radio active decay), pulsars (fast spinning (600 rps) of huge objects), afterglow (cooling gasses), jets (result of black hole annihilations)
Nuclear power, radioactive material, particle accelerators
This band of the spectrum is most crowded with uses for 1001 devices such as:
Mobile phones, satellite communications, radar, TV, radio, ..
Interaction with matter:
Kills or mutates living cells