The main fragmentation option in PYTHIA is the Lund string scheme, but some simple independent fragmentation options are also available. These latter options should not be taken too seriously, since we know that independent fragmentation does not provide a consistent alternative, but occasionally one may like to compare string fragmentation with something else.
The subsequent four sections give further details; the first one on flavour selection, which is common to the two approaches, the second on string fragmentation, the third on independent fragmentation, while the fourth and final contains information on a few other issues.
The Lund fragmentation model is described in [And83], where all the basic ideas are presented and earlier papers [And79,And80,And82,And82a] summarized. The details given there on how a multiparton jet system is allowed to fragment are out of date, however, and for this one should turn to [Sjö84]. Also the `popcorn' baryon production mechanism is not covered, see [And85], and [Edé97] for a more sophisticated version. The most recent comprehensive description of the Lund model is found in [And98]. Reviews of fragmentation models in general may be found in [Sjö88,Sjö89].