Computers have been designed to discover eachother on local networks. It's harmless.
As long as you have file sharing and screen sharing turned off (System Preferences > Sharing) there's no way your roommate could see what's inside your computer.
That depends on if your roomate has the password to your user account (or if you have not set a password!).
Other apps have sharing features too. If you have iPhoto sharing active they may be able to view photos etc, other apps also advertise the ports for users to connect to.
This claims to disable the bonjour advertisements…
I don't know if it is still valid for 10.9, but it was updated this April. It doesn't make the Mac any more secure, it simply stops the advertisements - any active sharing services will still accept valid connections, but you will require manual setup (via IP address or hostnames).
It may be a good idea to change your 'computer name' in sharing if you use open wifi a lot. Seeing 'John Smiths macbook pro' on free wifi makes me chuckle.
P.S. Bonjour is a 'local network thing' it shouldn't be visible on the internet (at least without some elaborate network setup).
File Sharing can be On or Off.
In Options you can turn On SMB (windows) sharing.
You decide which files/Folders can be Shared in which Mac User accounts
(They may have an account on your computer which they might legitimately access)
You can decide on Who has access.
You can also decide if people can Read (open with an App on their Computer) or Write to the File ( Save a new version on your computer).
If they do have access to Files inside a Folder they may still need Write Permission to the folder as well as the File itself.
10:17 pm Saturday; May 3, 2014 iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad