Gamma rays are a form of light.
What humans think of as "light" is only a small part of a vast spectrum of electromagnetic energy.
When a body possesses an electric charge some of its atomic particles will emit energy by giving off photons - quantum particles of electromagnetic radiation. These travel through the universe at the speed of light, 186 000 miles per second. The only things able to travel at this speed, photons are thought to be massless.
Light behaves as both a wave and a particle, so scientists talk of the frequency of light waves. Low frequency waves are long, low energy waves; high frequency waves are short, high energy waves. Radio waves are the longest form, sometimes with wavelengths of many metres; then microwaves, then infrared; then visible light. As we progress from radio waves, the wavelengths are decreasing and the energy is increasing. Infrared waves are often wrongly said to be "heat" - all electromagnetic radiation can be felt this way, on some level, but infrared cameras can detect heat sources such as animal bodies (the police use them to catch escaping criminals at night).
When people see things, it is because visible light is being given off by some source and reflected off various objects. Our eyes can detect visible light, but they cannot detect any other form of light. Going further along the spectrum towards the shortest wavelengths and highest energy, there are ultraviolet waves, x-rays and finally gamma rays.
Gamma rays therefore are the highest frequency, shortest wavelength (between 3/100ths and 3/1000ths of a nanometer itself a billionth of a millimetre), highest energy type of light wave there is. One difference in the emission of gamma rays and x-rays, which are similar but have lower energy, is that gamma rays are given off by the nuclear particles of an atom - a neutron or a proton, while x-rays come from electrons.
In fact, gamma rays are generally given off by the strongest and most powerful sources of energy. The centres of nuclear explosions; the centres of supernova explosions; collisions between neutron stars; black holes. These gamma ray bursts occur at intervals throughout the universe, and when they do they give off more energy in a few minutes than their entire galaxy does in the same period.
Gamma rays are also emitted in the process of radioactive decay, which is how they were discovered by Henri Becquerel in 1896.
Because they are sometimes given off by naturally-occurring substances (known as radionuclides), gamma rays are present in various forms all the time. Some radionuclides are found in soil and even in some fruits (such as bananas)! These quantities are very small and are not dangerous.
The high energy nature of gamma rays from strong sources such as nuclear explosions means they are ionizing radiation. They have the energy to split electrons from atoms in the molecules they encounter - the resulting, changed atom is known as an ion. The electron is free to move - a free radical. These can damage tissue. The main effect of this is cancer. So gamma rays are extremely dangerous to the human body and need to be shielded in laboratories with lead.
Ironically, this has led to gamma rays being used as a treatment for cancer, as well as for sterilising hospital equipment. Gamma ray emitting substances are also used widely for making materials such as wood and plastics stronger. This is due to the way the gamma rays affect the structure of those materials.
Gamma ray bursts have been advanced as a possible cause of extinctions on Earth. If there were a nearby supernova (say only a few thousand light-years away) and if the star were aligned suitably with Earth, it could send a burst of gamma rays towards us. This burst could strip the ozone layer away, resulting in higher levels of ultraviolet light, which would be fatal for many species of plants and animals. This is a hypothesis, but the science is sound.