As the remaining frontier for increasing the communication capacity of optical fibers, multi-core fibers with space division multiplexing have attracted much attention in recent years, with many solutions proposed for expanding the capacity and improving the transmission speed. This paper comprehensively reviews the basic properties of multi-core optical fibers, especially in non-optical-fiber communications and sensing applications. We focus on the main issues related to multi-core fibers, namely, inter-core coupling and crosstalk problems, the transmission and bending characteristics of the fibers, fan-in/fan-out and connection technologies, the taped fiber coupling technique for multicores, and other dominant problems in optical fiber communication technology. A new hollow elliptic multi-core polarization-maintaining fiber is discussed as well. In a series of examples, we finally show how multi-core fibers are applied in high-power fiber lasers and fiber interference, emphasizing their applications in various sensing fields.