Apologies, I wasn't intending it as rude, I was just trying to get across what it meant technologically.
" allow half the web to use Gmail, Twitter, LinkedIn, or other trusted servers to log into completely different services."
Again, not intended as rude, but I think there might be some disagreement there over what constitutes trusted.
I would never use Google or Twitter or Microsoft accounts to log in on another service, and they are all centralised services.