As the founder of bitcoin-otc, I think I'll be able to help you out here. :)
OTC uses gpg (the free software implementation of the PGP protocol) for authentication. So your first step is to register on OTC with your gpg key. The OTC wiki contains detailed instructions on this page: http://wiki.bitcoin-otc.com/wiki/GPG_authentication
The rating system is a web of trust links between users. There is more documentation on the rating system here: http://wiki.bitcoin-otc.com/wiki/OTC_Rating_System . I suggest that you pay particular attention to the 'relying on the rating' section.
Trades happen p2p - the OTC system is only an order aggregator as far as trading is concerned. Once orders are placed (which is not even a requirement, you can just hang out on the IRC channel and set up trades without ever using the order book), it is up to the individuals to talk to each other and set up the trade details.
From the order book view, you can see a person's ratings by clicking on his/her username.
The bitcoin-otc system is not a for-profit entity, nor a profitable one. There are no fees charged by the system for users to register and place orders. All order execution happens directly between people, the otc system does not intermediate either side of the transaction.
Overall, there is plenty of documentation on the OTC website and wiki; I invite you to give it a read for further details. A good starting point would be the main user guide page: http://wiki.bitcoin-otc.com/wiki/Using_bitcoin-otc