Short answers, pulled from the story.
Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. He noticed it was more powerful than previously described rays but did not name it as a new fundamental type.
Ernest Rutherford named them gamma rays in 1903 based on their strong penetration of matter. He used the first three letters of the Greek alphabet for decay radiation in ascending order of penetrating power, with alpha as least penetrating, then beta, then gamma as the most penetrating.
Gamma rays and X-rays overlap in energy, so they are now usually distinguished by origin rather than wavelength. X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus or by particle decays and annihilation events. In astronomy, gamma rays are conventionally defined as having photon energies above 100 keV.
Gamma rays are best absorbed by dense materials with high atomic numbers, such as lead or concrete. Gamma rays that need 1 cm of lead to halve their intensity are also halved by 6 cm of concrete or 9 cm of packed soil. Unlike alpha particles, which paper stops, and beta particles, which thin aluminium stops, gamma rays require large amounts of shielding mass.
Yes, gamma rays are ionizing radiation hazardous to life, capable of causing DNA mutations, cancer, tumors, burns, and radiation sickness. An acute full-body dose of 3 to 5 Sv is about the LD50, the level lethal to half of those exposed even with standard medical treatment, and doses above 7.5 to 10 Sv are not survivable even with bone-marrow transplants.
Gamma rays are used to treat cancer through gamma-knife surgery, in which multiple concentrated beams converge on a growth from different angles to kill cancer cells while sparing surrounding tissue. They also sterilize medical equipment and enable diagnostic imaging using the nuclear isomer technetium-99m, which emits gamma rays at 140 keV.
The gamma ray sky is dominated by pulsars within the Milky Way, with most other sources being quasars. The most intense sources are gamma-ray bursts, whose long-duration form releases about ten to the forty-fourth joules in only 20 to 40 seconds and can be detected up to 10 billion light years away.