The smart things hub can be thought of as the brain for all your smart home devices to be controlled. It integrates with a large number of devices and is easily set up. The cost of the hub is $99.
Samsung includes on its smart things website a comprehensive list of all kinds of smart home products that work well with the smart things hub. It includes sensors, cameras, switches, lights and all the home automation tools you would expect. Some smart home hubs are limited in its ability to integrate easily with other products, but the samsung one is by far the most versatile. For this reason its one of the most popular as well. You can see the list here: https://www.smartthings.com/works-with-smartthings
Also popular with the samsung hub is the smart things monitoring kit. It provides a basic set of sensors to be added into your home for detecting motion, controlling an outlet, and 2 multi purpose sensors.