Thanks for your interest in MultiBit.
MultiBit is based on the bitcoinj library and the wallet format is different to the Satoshi client wallet.dat
It is not currently encrypted though that is on the development plan.
There is quite a lot if explanation of what MultiBit does on it's bitcointalk thread:https://bitcointalk.org/index.php?topic=43616.0
The website http://multibit.org
also has general information but is deliberately nontechnical.
If you want the full technical details the code is open source and is available at https://github.com/jim618/multibit