  The demanding objectives for the future sixth generation (6G) of wireless
communication networks have spurred recent research efforts on novel materials
and radio-frequency front-end architectures for wireless connectivity, as well
as revolutionary communication and computing paradigms. Among the pioneering
candidate technologies for 6G belong the reconfigurable intelligent surfaces
(RISs), which are artificial planar structures with integrated electronic
circuits that can be programmed to manipulate the incoming electromagnetic
field in a wide variety of functionalities. Incorporating RISs in wireless
networks has been recently advocated as a revolutionary means to transform any
wireless signal propagation environment to a dynamically programmable one,
intended for various networking objectives, such as coverage extension and
capacity boosting, spatiotemporal focusing with benefits in energy efficiency
and secrecy, and low electromagnetic field exposure. Motivated by the recent
increasing interests in the field of RISs and the consequent pioneering concept
of the RIS-enabled smart wireless environments, in this paper, we overview and
taxonomize the latest advances in RIS hardware architectures as well as the
most recent developments in the modeling of RIS unit elements and RIS-empowered
wireless signal propagation. We also present a thorough overview of the channel
estimation approaches for RIS-empowered communications systems, which
constitute a prerequisite step for the optimized incorporation of RISs in
future wireless networks. Finally, we discuss the relevance of the RIS
technology in the latest wireless communication standards, and highlight the
current and future standardization activities for the RIS technology and the
consequent RIS-empowered wireless networking approaches.
