Tor isn't a browser, they just bundled it with a browser to help the newbies.
The key to Tor is the onion routing scheme. One way to think of onion routing is envelopes in envelopes. Take your post to RIU and place it in a series of envelopes, each one with a different address on it. These envelopes have a special seal that only the intended recipient can remove. You send the envelopes in the mail, and the mail courier doesn't know what is in the envelope or where it is really going. It gets delivered to the first recipient who opens the outermost envelope. This person doesn't know what is in the second envelope or where it is really going. They send the envelope onto the next person, who knows even less then the first recipient. They don't know what is in their envelope, where it really came from, or where it is really going. The last person to receive your post opens the last envelope, seeing both your post and the actual destination of your post. They don't (directly) know the source of the post. If you were posting to a website inside the Tor network the last person would be that website not a middleman.
Tor wraps your communication with several layers of encryption. It is then sent to an internal relay on the network who is able to remove the "outermost" layer of encryption and send it on to the next relay. The last relay in the chain is either the hidden service you are communicating with (for example Silk Road) or the exit node that removes the last layer of encryption and sends it out into the clearnet (for example RIU.) Tor frequently changes the route used to further obfuscate the communications. When dealing with clearnet the exit node is a serious problem. Without encryption and authentication the end node can spy on and modify your communications. RIU provides no protection against this (doesn't offer TSL encryption let alone force it.)
Assuming you don't do something stupid Tor provides good protection within the network. It's not bullet proof but it's seldom the weakest link in the chain.
