The image below shows the different types of devices in a Thread Network.
Routers provide routing services to network devices. Routers also provide joining and security services for devices trying to join the network. Routers are not designed to sleep. Routers can downgrade their functionality and become REEDs (Router-eligible End Devices). A Router can become a Leader and start a Thread network.
A Border Router is a type of Router that provides connectivity from the 802.15.4 network to adjacent networks on other physical layers (for example, Wi-Fi and Ethernet). Border Routers provide services for devices within the 802.15.4 network, including routing services for off-network operations. There may be one or more Border Routers in a Thread Network.
The Border Router also serves as an interface point for the Commissioner when the Commissioner is on a non-Thread Network; it requires a Thread interface and may be combined in any device with other Thread roles except the Joiner.
A Router or Border Router can assume a Leader role for certain functions in the Thread Network. This Leader is required to make decisions within the network. For example, the Leader assigns Router addresses and allows new Router requests. The Leader role is elected and if the Leader fails, another Router or Border Router assumes the Leader role. It is this autonomous operation that ensures there is no single point of failure.
Router-eligible End Device
REEDs have the capability to become Routers but due to the network topology or conditions these devices are not acting as Routers. These devices do not generally forward messages or provide joining or security services for other devices in the Thread Network. The Thread Network manages REEDs becoming Routers if necessary without user interaction.
Sleepy End Device
Sleepy end devices are host devices. They communicate only through their Parent Router and cannot forward messages for other devices