Ella - this site:
http://arstechnica.com/paedia/w/wire...802.11b-1.html
has a good review of what you should do (in pretty simple terms).
Basically, the wireless router (or access point) can be connected to by any wireless device (yours and your neighbours). By turning on the security features of the router, you will need to do some config on your pcs to connect but it means that your neighbours would also need to do the same config to their machine to connect as well. In essence you have the key and they do not.
I'm not a novice in this area, but I'm also not an expert. If we can't solve it here, wander over to the computer forum - there are piles of nerds (...I mean experts) there
