Ripple is not centralized.

Using my (overly simplified) agreement room analogy, it works like this:

1) There's a room where everyone agrees on the last closed ledger.

2) If you want to disagree, you can, but you must leave the room to do so.

3) If you want to know what the current ledger is, you walk in and ask everyone in the room.

4) If you want to perform a transaction, you read it out loud. Everyone honest agrees that it's valid.

5) If there are any transactions that someone in the room believes is valid and is not in the current ledger, they attempt to obtain a consensus on that transaction.

6) When a consensus is reach, the consensus transactions are applied to the last closed ledger forming a new last closed ledger.

7) Everyone agrees on the new last closed ledger.

8 ) We go back to step 5. Any transactions believed still valid but that didn't get into the consensus transaction set for some reason should now be voted in by every honest person.

9) If people appear to be acting in ways that don't make sense, such as voting no on transactions that have no reason not to be included or failing to validate the correct ledger, you ignore them.

10) Your top priority is to enforce the rules of the room. Your second priority is to achieve consensus.