Smart sensors and network systems are commonly referred to as Internet of Things (IoT) and are being used to realize a smart society. Although the development of low-power smart systems and large-capacity batteries is increasing the usage time of IoT devices, the time-limited capability of such systems reveals a need for self-powered sensors and systems for sustained IoT use. Mechanical energy is easily accessible from the environment to power sensors and systems. The triboelectric nanogenerator (TENG) converts mechanical energy into electric energy was first introduced in January 2012 by Wang et al., and we describe recent developments in the triboelectric properties of the polymers because the contact electrification between the two different materials is a key factor of TENG. This review article discusses the four operating modes of TENG, the working mechanism, the theoretical modelling of the vertical TENG, and the research aspects of the material.