What is it ?
Amateur Radio Emergency Data Network
AREDN is an evolution of HSMM (High Speed Multimedia Radio). It uses the OLSR routing protocol to dynamically mesh any nodes on the same frequency. It's topology is very much like traditional packet radio where all stations in a given area are typically on the same frequency. It gets around some of the issues that packet radio had by being high speed (>5Mbps).
There is no requirement that an AREDN network have any connectivity to the internet. The fact that this is done is usually for convenience in connecting HAM's home networks in the shack to the ARDEN without interrupting their own connections to the internet and also for the purpose of connecting AREDNs together via VPN tunnels through the internet where an RF link isn't feasible.