Now that is scary, as I don't want to give all my personal data to some random company's server in Europe? Firefox is cool with me but Telefonica, not so much.
In general, it's peer-to-peer, but for clients that can't communicate peer-to-peer for whatever reason (usually due to certain NATs), a relay server is used as an intermediary. To the user it's transparent, so they can't easily tell if it's peer-to-peer or not.