The advent of power line communication (PLC) for smart grids, vehicular communications, and data network access, has recently gained ample interest among the researchers and industries. Couplers - devices used to inject data communication signals into (Alternating Current or Direct Current) power lines are very important components of a PLC system. There is, however, an obvious gap in literature for a detailed review of existing PLC couplers. In this paper, we present a comprehensive review of couplers, which are required in narrowband and broadband PLC transceivers. We provide, also, a novel classification of PLC couplers based on the type of physical coupling, voltage levels, frequency bandwidth, propagation mode, and number of connections. Prevailing issues that protract the design of couplers and consequently subtended the invention of different types of couplers are clearly described. This survey will guide researchers, and designers alike, into a quicker resourcing when studying coupling in narrowband and broadband PLC systems.