  The hadronization of a high-energy parton is described by fragmentation
functions which are introduced through QCD factorizations. While the
hadronization mechanism per se remains uknown, fragmentation functions can
still be investigated qualitatively and quantitatively. The qualitative study
mainly concentrates on extracting genuine features based on the operator
definition in quantum field theory. The quantitative research focuses on
describing a variety of experimental data employing the fragmentation function
given by the parameterizations or model calculations. With the foundation of
the transverse-momentum-dependent factorization, the QCD evolution of leading
twist transverse-momentum-dependent fragmentation functions has also been
established. In addition, the universality of fragmentation functions has been
proven, albeit model-dependently, so that it is possible to perform a global
analysis of experimental data in different high-energy reactions. The
collective efforts may eventually reveal important information hidden in the
shadow of nonperturbative physics. This review covers the following topics:
transverse-momentum-dependent factorization and the corresponding QCD
evolution, spin-dependent fragmentation functions at leading and higher twists,
several experimental measurements and corresponding phenomenological studies,
and some model calculations.
