It has been a slow transition, but Skype is finalizing its move away from a peer-to-peer system to a cloud-based one.
When it was first created, the Skype network was built as a decentralized peer-to-peer system. PCs that had enough processing power and bandwidth would be elected as “supernodes” and used to coordinate connections between other machines on the network. Similarly, text, voice, and video traffic would flow between peers, directly when possible (when intervening firewalls and routers were cooperative) or indirectly through other systems on the network when required.
