At the heart of the relaxor behavior lies the development of mesoscopic or intermediate range order and the low frequency or relaxation dynamics associated with it. To investigate this development, we have measured the Raman spectra of a (PZN) single crystal over a wide temperature range from . The spectra are analyzed using several different physical models, focusing particularly on the low frequency part. Both relaxation and coupled phonon dynamics are identified. The evolution of the relaxor dynamics is marked by three characteristic temperatures, Burns temperature, , and two other temperatures, and , which together define four major ranges. Combining the fitting results from the Raman spectra and other experimental results in PZN, we describe the relaxor dynamics in these four ranges, in terms of the strength and lifetime of the correlations between off-center ions. Relating the dynamics observed in PZN to that in (KTN), we develop a microscopic model based on the coexistence of a fast and a slow motion of the off-center Pb and Nb ions which accounts for both types of dynamics (coupled phonon and relaxational) between and , and show evidence for a series of local phase transitions taking place below .