In this paper, we attempt to reveal the nature of the chemical and electromagnetic mechanisms of tip-enhanced Raman scattering (TERS). Direct visual evidence regarding the chemical mechanism via charge transfer was obtained with charge difference density. It is found that there are several kinds of charge transfer: (1) tip to molecule, (2) surface to molecule, (3) tip and surface to molecule simultaneously, and (4) tunneling charge transfer between the tip and the surface. Direct evidence regarding the electromagnetic mechanism via intracluster (tip or surface) charge redistribution was also revealed via charge difference density. The distance (between tip and surface) dependence of the near electric field distribution and the TERS enhancement at different incident lights is also discussed using the three-dimensional finite-difference time-domain (FDTD) method. The electromagnetic enhancement of double-tip TERS is approximately 10 times larger than that of conventional TERS. Theoretical results reveal that plasmon coupling effects between the metal tip and surface play an important role in TERS.