‘Generalized synchronization (GS)’ was proposed by Rulkov et al. (Phys. Rev. E 51, 980 (1995)) to explain synchronization in unidirectionally coupled systems. This concept has been effective in providing a deeper understanding of synchronization between several non-identical nonlinear systems. The study of GS has been extended to various coupling schemes ranging from bidirectionally coupled systems to complex networks. We review the major ideas that were involved in the field of GS. The characterizing tools to detect generalizedsynchrony and its types along with their relative merits are discussed. We outline some of the interesting results that were published in the last two decades along with the inter-relatedness of different dynamical phenomena reported in coupled dynamical systems.