What service does Multibit connect to?
Multibit uses SPV, which is a peer-to-peer protocol consisting of two parts.


It downloads the block headers from peers (not the whole block). This allows it to verify that the blocks have a lot of hashing power behind them, and get the merkle root for each of them.
It asks peers for transactions matching the addresses it cares about. It can verify that the transactions belong to one of the blocks it knows about by using the merkle root.


See here for a discussion of the security tradeoffs.