Wireless networks are less secure than cable ones. If you do not secure your wireless network then anyone within range can access it.
The importance of security
When you connect to your own wireless network you will probably see all of the others within range in a pop up like this one. Just as you can see othetr people's networks, they can see yours and, if it is not secure, they will be able to use your internet connection - at your cost. Even if you trust your neighbours there are websites…