First discovered by accident by military satellites whose main mission was to detect space-based nuclear explosions, gamma ray bursts are highly energetic explosions that astronomers have yet to fully understand. Collisions between black holes or neutron stars have the potential to release such massive amounts of energy, but it isn't known if this is the cause of the bursts.
They were first observed in the mid-1960s but were not reported in scientific journals until 1973. Because Earth's atmosphere filters out gamma radiation (thank goodness!) bursts can only be observed from space. NASA launched the Compton Gamma Ray Observatory in 1991 to study bursts among other phenomenon, but this satellite was purposefully de-orbited in June 2000.
Bursts occur on average once a day. They occur randomly in all directions. Often the bursts outshine all other gamma ray sources in the sky then fade away leaving no trace. Bursts are accompanied by energy in other wavelengths, but because the bursts are so fleeting and often extremely distant, observing bursts at other wavelengths is quite a challenge, especially at when they are at their brief peak intensity.
In 1998 astronomers detected a burst which turned out to be the most energetic event in the Universe since the Big Bang. Occuring 12 billion light years away (only a couple billion years younger than the Universe itself), the explosion was several hundred times bigger than a typical supernova, which was the old benchmark for big explosions.
They remain one of the great unexplained questions in modern science.