In physics, gluons are the bosonic particles which are responsible for the strong nuclear force. They bind quarks together to form protons and neutrons as well as other hadrons; their electric charge is zero, their spin is 1 and they are generally assumed to have zero mass (although a mass as large as a few MeV may not be precluded). Gluons are ultimately responsible for the stability of atomic nuclei; there are eight different kinds of gluons.
In quantum chromodynamics (QCD), today's accepted theory for the description of the strong nuclear force, gluons are exchanged when particles with a color charge interact. When two quarks exchange a gluon, their color charges change; the gluon carries an anti-color charge to compensate for the quark's old color charge, as well as the quark's new color charge. Since gluons thus carry a color charge themselves, they can also interact with other gluons, which makes the mathematical analysis of the strong nuclear force quite complicated and difficult.
The first experimental traces of gluons were found in the early 1980s at the large electron-positron collider PETRA at the DESY in Hamburg, when evidence for a clear three-jet structure was found; the third jet was attributed to one of the produced quarks emitting a gluon.
Why are there only 8 gluons?
From a layman's point of view, there theoretically are nine unique color combinations for gluons, one for each combination of color (red, green and blue) and anti-color: .
In mathematical terms, however, there actually exists an infinite number of gluons, each of them being a normalised linear superposition of the ones listed above (for example, etc). Furthermore, since experiments show that colorless baryons do not interact, the following property must hold; if it did not, baryons would be able to emit these gluons to interact with each other via the strong force.
No other linearly independent relation exists between gluons; thus, there are only eight linearly independent gluons, a statement that is typically simplified to "there are only 8 gluons". For a more detailed explanation involving SU(3) symmetry, see  (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/gluons.html).