A substantial amount of growth is being achieved by businesses through IoT-based services. The emergent of small electronic devices capable of computing, which are commonly known as wearables in IoT domain has proven to have huge impact in people's life. Theses wearables are capable of collecting vital information about a person's activities and behaviours regularly. This makes them suitable for many applications in health monitoring, fitness, sports, education and some industry related applications. To this end, in this paper, we aim to provide a general review on IoT-based wearables, the sensors adopted for several categorized wearables, the communication technologies adopted and the most widely adopted data processing techniques for wearables. Furthermore, we present the challenges faced for wide adoption of wearables and the future research directions.