The gluon is the carrier of the strong nuclear force
. The strong force has something analogous to electric charge
, but there are three types, called Red, Green, and Blue, along with the corresponding anti-colors. Each gluon has one unit of color and one unit of anti-color.
Since gluons carry strong charge they can interact with each other (unlike photons), and perhaps form glueballs.
If you count the combinations there should be nine types of gluons, but emprical evidence (largely from particle accelerators) suggests that there are only eight. This expresses the fact that Nature is colorless, i.e. all strongly interacting particles (except virtual particles) must form combinations that have equal ammounts of color and anti-color (e.g. one red and one anti-red), or must have equal ammounts of all three colors (one red, one blue, and one green). This accounts for the fact that no free quarks have ever been observed.