The fifth generation (5G) system was marketed as a true enabler of latency-aware ultra-reliable low latency communications (uRLLC), bandwidth-devouring enhanced mobile broadband (eMBB), and unlimited-things-centric massive machine type communication (mMTC) services for vertical industries and internet-of-everything (IoE). Among other promises, the eMBB services have gained significant momentum in various 5G deployments, such as using millimeter wave (mmWave) frequency band in the next-generation radio access network (NG-RAN). The early rollouts of the uRLLC and mMTC services show that the 5G system supports their basic features, however, supporting advanced and complicated use-cases of such services remains uncertain. These drawbacks are due to, but not limited to, the existing limitations in the adaptation of current technologies, such as multi-access cloud computing, network function virtualization, software-defined networking (SDN), network slicing, etc. in NG-RAN. In this regard, we aim to (i) provide a comprehensive overview on key concepts towards the RAN of sixth generation (6G) system; (ii) study various legacy RAN implementations, reported in literature, from different perspectives, and the motivation of their reconstruction and redesign with respect to the requirements of their forth-coming generations; (iii) present an exclusive review of the NG-RAN with special emphasize on the cloudification and virtualization of resources and services, and on the management and orchestration of RAN slices; (iv) identity the key drivers that leverage advanced mMTC, uRLLC, and eMBB services and applications; and (v) address key challenges and future research arising from the deployment of NG-RAN.