Here is how it works.
If you get a cable/dsl modem from your provider that has WIFI built into it, then it is possible that it is broadcasting this connection to the public. I know that Time Warner Cable does where i live. Others can connect to it if they have the services themselves and yes, they do use your bandwidth that you pay for. When i help family members set their stuff up, i disable this option as i am not fond of others using my bandwith.
If you have a simple modem that does not support wifi and provide your own Wireless router, then you are not sharing this capability.