A brief theory overview on quark-gluon plasma (QGP) produced in relativistic heavy-ion collisions is presented. Thermal statistical model and hydrodynamic model have been shown to be successful tools to study hadron spectra at low (soft) transverse momentum; while perturbative QCD calculation, corrected with non-perturbative interactions between partons and a thermal medium, has been found essential in understanding heavy quark and high transverse momentum jet (hard) spectra. Using these sophisticated model calculations, reasonable extractions of various properties of the QGP have been achieved, such as its initial geometry, transport coefficients and equation of state. Better constraints on additional information, such as in-medium hadron structure, evolution profile of electromagnetic field and pattern of jet-induced medium excitation in high-energy nuclear collisions, are expected with more precise measurements at LHC and RHIC in the near future.