what is the algo they use to do this, how does the consensus work....it seems fast.
centralization.
Nope, Ripple is not inputs.io.
https://ripple.com/wiki/ConsensusXRP were issued to one single key/person/entity, they can be owned once you get someone to transfer them to you, but there is no proof of work or similar approach to distribute them in any way.
who decides who gets on the UNL....
Each validator operator themselves.
seems that a whole load of servers couls sit there and ban out the other servers saying the yare not on the UNL and rewrite what the wish, so a 51% by server....or something similar
Then they just risk creating their own fork... as long as they do not make up the majority (actually >80%) of the UNLs on all other nodes, there is little chance for such an attack to succeed. It would be very easy to proof that someone tried this type of attack though, since validators need to sign ledger headers with their private key. This is a bit similar to PoS maybe, just that there is no weighting according to "Stake".
