Guess you don't have wireless router

if you have subscribed the internet connection service, you will have a modem, then after the modem, you can connect your PC. Now a lot of people have more than one computers at home, plus wireless notebooks, so they use the router (or wireless router), and let all computer at home to share one internet connection. For wireless router, how to make sure your connection not used by unauthorized users:

1. make your own SSID not broadcast, there is the default setting from the manufactures, e.g. for Linksys, its default is "linksys", you can change this as anything you want, plus not easily guessed.
2. supposed others know your SSID, you can set the router to ask the passphase (like a password),
3. or set MAC code access, only allow the certain MAC (each computer has unique MAC number) to connect your wireless router.
4. set signal encrypted if need to prevent from interception , this will slowdown the speed. I do not use this.

If your office has the wired internet connection, you do not need the wireless, although your notebook can detect the signals (you can turn off the wireless). The people also could change their settings of their router at any time. So you may be used, but now could not.

Actually, you can find a lot of wireless signals for free, because there are the people who do not do setups of their routers. they just connect and use, who cares! so if you see SSID listed, you can try if you can connect to, no need to try not named SSID signals.