Gamma rays, x-rays, visible light, microwaves and radio waves are all examples of electromagnetic radiation.  The distinction between the different types is in the wavelength.  Radio waves are longer than light waves and gamma rays have the shortest wavelength of all.  This means that gamma rays have more energy.  A photon of visible light may possess about 2 electron volts (eVs), an x-ray photon may have 50,000 eVs, but a gamma ray photon may possess over 200,000  or even upwards of a billion eVs.  They are very dangerous for the living.

Gamma rays are a consequence of subatomic interaction.  For example a relativistic electron may result in an electron / positron pair.  A positron has the same mass as an electron but has the opposite charge.  When an electron and positron meet they annihilate each other and emit two gamma rays each of which travels in opposite directions.  A pion is also a subatomic particle found in the nucleus of an atom.  If it should leave the nucleus it almost immediately transforms itself into gamma radiation.  The neutral pion is it’s own antiparticle.  The negative pion is the antiparticle of the positive pion.  A pion has more mass than an electron, but less mass than a proton.  The conversion of mass to energy follow Einstein’s formula E = MC².  Therefore, even a little mass can produce a respectable amount of energy.

Gamma ray burst (GRB) is just a burst of short lived gamma rays photons.  GRB’s are associated with many astronomical phenomena such as supernova, which is the most massive explosion known and which occurs at the death of a star.  Mystery still surrounds GRB’s as they are still not clearly understood by scientists.  Once GRB’s were only associated with supernova, but more recently they have been discovered in our own atmosphere.  So the question is what creates these bursts of high level energy?

Whether the GRB’s last a few milliseconds or several minutes, they are about a million trillion times as bright as our Sun.  Viewed from our planet, somewhere in our universe a GRB is seen at least once a day.  Those seen are only a small fraction of the total.  The bursts are pin points of light that must be pointing directly to the Earth in order to be seen by someone on the Earth.

Possibly the annihilation of a particle and antiparticle create not only gamma rays but also a blast wave.  The energetic gamma rays can outrun the blast wave so they get out ahead of it.  Meanwhile,  as the blast wave pushes outward it encounters matter which gets converted to x rays and visible light rays, and this lower energetic electromagnetic radiation is known as the afterglow.

Reference:

http://en.wikipedia.org/wiki/Gamma_ray_burst