The major principle at the heart of quantum chromodynamics is that there are six types of quarks, and they interact in different ways. Quarks can be bound together by the strong nuclear force, which manifests in the form of a gluon, the boson particle that mediates the strong nuclear interaction. (Gluons can also join together without quarks, in objects called "glueballs.")
The theory of quantum chromodynamics was developed through the 1950s and 1960s, resulting in a 1969 Nobel Prize in Physics for Murray Gell-Mann (although asymptotic freedom was discovered later by David Politzer, Frank Wilczek, and David Gross, who split the 2004 Nobel Prize in Physics for this work). Experimental research since then has strongly confirmed the theory, and the six flavors of quarks predicted by the theory have all been observed experimentally in laboratory settings.
The name quantum chromodynamics comes from the naming convention that the quarks have a "color," which acts sort of like a "charge" in electrodynamics (but has nothing to do, except in name, with visible colors). "Chromo" is Greek for "color," so the name comes from the way these "colors" interact with each other.