An IoT OS is an operating system that is designed to perform within the constraints that are particular to Internet of Things devices, including restrictions on memory, size, power and processing capacity. IoT operating systems are a type of embedded OS but by definition are designed to enable data transfer over the internet.
The essence of the Internet of Things (IoT) is the ability of embedded systems all around us to connect and communicate over a network. IoT OSes control systems in cars, traffic lights, digital televisions, ATMs, airplane controls, point of sale (POS) terminals, digital cameras GPS navigation systems, elevators, digital media receivers and Smart meters among many other possibilities. The variety from one host to another means that IoT operating systems have a similar variability in requirements.
A sampling of IoT OSes:
Ubuntu Core is a transactional version of the Ubuntu Linux OS, made specifically for IoT devices and large container deployments.
RIOT is an open source OS specialized for IoT devices. RIOT was developed by a world-wide grassroots community of companies, academics and hobbyists.