The kaon (or K-meson) is a kind of sub-atomic particle consisting of some combination of a strange quark, and an up or a down quark (one of these must be a matter quark and one an anti-quark). They are spin-1 particles that obey Bose-Einstein statistics, and each have a rest mass of approximately 495 MeV/c2.
Kaons have been of particular interest to physicists since the early days of high energy physics (c. 1964) because they represent one of the first systems to ever be observed demonstrating the phenomenon of CP violation. This specific kind of symmetry-breaking seen is considered to be one of the most interesting possible explanations for the matter (as opposed to antimatter) dominated world in which we live.
There are four different kinds of kaons: the K+ (up and anti-strange), the K- (strange and anti-up), the K0 (down and anti-strange), and the anti-K0 (strange and anti-down).