QCD perturbation theory, formulated in terms of quarks and gluons, is valid at short distances. At long distances, QCD becomes strongly interacting and perturbation theory breaks down. In this confinement regime, the coloured partons are transformed into colourless hadrons, a process called either hadronization or fragmentation. In this paper we reserve the former term for the combination of fragmentation and the subsequent decay of unstable particles.
The fragmentation process has yet to be understood from first principles, starting from the QCD Lagrangian. This has left the way clear for the development of a number of different phenomenological models. Three main schools are usually distinguished, string fragmentation (SF), independent fragmentation (IF) and cluster fragmentation (CF), but many variants and hybrids exist. Being models, none of them can lay claims to being `correct', although some may be better founded than others. The best that can be aimed for is internal consistency, a good representation of existing data, and a predictive power for properties not yet studied or results at higher energies.