Wirelesss connection and SAFETY.?

My sister told me that she's been using her neighbor's wireless connection to surf the web, and the neighbor doesn' know about it. I've heard that the OWNER of the wireless connection would be able to somehow "see" your computer on the network and your private information if you use their unsecured network. Is this true? ANd if so, what KIND OF information of hers would the owner be able to see? THANKS.