Email is federated (you pick a mail host and can communicate with people on different hosts), while BitTorrent is distributed (users send data to each other without needing a fixed set of servers to mediate).
Technically, decentralisation refers to anything that is not centralised (so both federated and distributed protocols would count) but in modern usage decentralisation refers more to distributed protocols.
Technically, decentralisation refers to anything that is not centralised (so both federated and distributed protocols would count) but in modern usage decentralisation refers more to distributed protocols.